EJOl-OGY 

LIBRARY 

G 


HUMAN  PHYSIOLOGY 

A  Text-Book  for  High  Schools  and  Colleges 


BY 
PERCY  GOLDTHWAIT  STILES 

ASSISTANT  PROFESSOR  OF  PHYSIOLOGY  IN  HARVARD  UNIVER- 
SITY; INSTRUCTOR  IN  PHYSIOLOGY  AND  PERSONAL  HYGIENE 
IN  THE  MASSACHUSETTS  INSTITUTE  OF  TECHNOLOGY  ; 
FORMERLY  ASSISTANT  PROFESSOR  OF  PHYSIOLOGY  IN 
SIMMONS  COLLEGE,  BOSTON 


ILLUSTRATED 


PHILADELPHIA  AND  LONDON 

W.   B.   SAUNDERS    COMPANY 
1916 

WWMiigWWWW^ 


S  g 


Copyright,  igi6,  by  W.  B.  Saunders  Company 


PRINTED     IN     AMERICA 

PRESS    OF 

W.     B.    8AUNOERS     COMPANY 
PHILADELPHIA 


TO  MY  FATHER 

EDMUND  ELY  STILES 

A  DAUNTLESS  OPTIMIST 


PREFACE 


TEXT-BOOKS  of  science  may  be  placed  in  two  classes. 
There  are  those  which  aim  at  fullness  of  statement  and 
seek  to  acquaint  their  readers  with  experimental  methods 
and  original  sources.  Such  books  must  give  a  large  place 
to  controverted  matters,  weighing  conflicting  evidence 
and  comparing  the  views  of  various  workers.  The  mak- 
ing of  them  is  properly  in  the  hands  of  great  masters  of 
the  several  branches. 

Other  books  have  a  more  modest  scope.  Their  pur- 
pose is  to  present  concisely  the  accepted  facts  with  only  a 
limited  description  of  the  experiments  by  which  these 
facts  have  been  established.  They  contain  compara- 
tively little  about  unsettled  questions  though  they  are  at 
fault  if  they  do  not  make  it  plain  that  these  confront  the 
investigator  at  every  turn.  They  may  be  written  by 
teachers  who  have  not  lost  the  point  of  view  of  elementary 
students  or  ceased  to  sympathize  with  them  in  their 
perplexities. 

The  present  book  belongs  definitely  to  the  second  class. 
An  extreme  course  of  action  has  been  adopted  with  regard 
to  the  accrediting  of  discoveries.  It  is  certainly  a  source 
of  irritation  and  bewilderment  to  the  beginner  to  have 
the  pages  he  reads  sprinkled  thickly  with  the  names  of 
men  of  whom  he  has  never  heard  before.  In  the  chapters 
that  follow  no  mention  is  made  of  living  experimenters 
though  a  few  eminent  physiologists  of  earlier  times  are 
referred  to.  It  has  been  hard  not  to  make  exceptions 
and  the  use  without  acknowledgment  of  illuminating 
ideas  and  happy  teaching  devices  which  I  owe  to  my  con- 
temporaries has  aroused  a  feeling  akin  to  guilt.  Some 
atonement  may  be  found  in  the  list  of  collateral  readings 
at  the  end  of  the  book. 

P.  G.  S. 

June,  1916. 

5 


TABLE  OF  CONTENTS 


PAGE 

To  THE  TEACHER 11 

CHAPTER  I 
INTRODUCTION 15 

CHAPTER  II 

PLANTS  AND  ANIMALS 25 

CHAPTER  III 
CELLS  AND  THEIR  ASSOCIATION 37 

CHAPTER  IV 
CONTRACTILE  TISSUES 53 

CHAPTER  V 
SKELETAL  MUSCLE 65 

CHAPTER  VI 

SKELETAL  MUSCLE  AND  THE  NERVOUS  SYSTEM 81 

CHAPTER  VII 

REFLEXES 94 

CHAPTER  VIII 
THE  BRAIN !    .    106 

CHAPTER  IX 

THE  BRAIN  (continued) — THE  CEREBRUM 121 

CHAPTER  X 

SENSATIONS  AND  THE  SENSE-ORGANS 136 

7 


8  TABLE    OF    CONTENTS 

CHAPTER  XI 

PAGE 

THE  EYE 149 

CHAPTER  XII 
THE  HYGIENE  OF  THE  NERVOUS  SYSTEM 163 

CHAPTER  XIII 
THE  ALIMENTARY  CANAL — DIGESTION 174 

CHAPTER  XIV 
SALIVARY  AND  GASTRIC  DIGESTION 188 

CHAPTER  XV 
INTESTINAL  DIGESTION — ABSORPTION 202 

CHAPTER  XVI 
THE  BLOOD 216 

CHAPTER  XVII 

THE  COURSE  AND  PHYSICS  OF  THE  CIRCULATION 227 

CHAPTER  XVIII 
THE  HEART 245 

CHAPTER  XIX 

THE  REGULATION  OF  THE  CIRCULATION 260 

CHAPTER  XX 
BREATHING 276 

CHAPTER  XXI 
RESPIRATION  (continued) 291 

CHAPTER  XXII 
METABOLISM  .  303 


PAGE 
.    315 


TABLE    OF   CONTENTS  9 

CHAPTER  XXIII 

EXCRETION 

CHAPTER  XXIV 

INCOME  AND  OUTGO 326 

CHAPTER  XXV 

THE  REQUISITES  OF  THE  DIET 340 

CHAPTER  XXVI 

THE  HYGIENE  OF  NUTRITION 352 

CHAPTER  XXVII 

THE  MAINTENANCE  OF  THE  BODY  TEMPERATURE 364 

CHAPTER  XXVIII 

INTERNAL  SECRETIONS  .    .    ,    .    .    .    •    •    •    •    •    » 374 

CHAPTER  XXIX 
SOME  MATTERS  OF  HYGIENE 384 

REFERENCES    ,        395 


INDEX 3" 


HUMAN  PHYSIOLOGY 


TO  THE  TEACHER 

It  is  commonly  assumed  that  the  chief  qualification  for 
the  teaching  of  physiology  is  a  knowledge  of  anatomy. 
Indeed,  what  is  called  physiology  in  the  lower  schools 
has  often  been  a  mere  description  of  the  organization 
of  the  body.  Anatomy  can  evidently  be  taught  without 
reference  to  other  sciences  and  the  instructor  who  follows 
the  line  of  least  resistance  with  regard  to  his  own  prepa- 
ration is  likely  to  give  a  large  place  to  the  pictorial 
branch.  But  the  temptation  is  one  to  be  resisted.  It  is 
a  sound  principle  to  subordinate  details  of  structure 
to  the  facts  of  operation. 

It  is  much  more  important  that  the  teacher  shall  be 
adequately  grounded  in  physics  and  chemistry  than  that 
he  shall  be  an  authority  on  anatomy.  The  difficulties 
experienced  by  the  student  are  not  connected  with 
shapes,  appearances,  and  arrangements  which  he  can 
visualize  but  with  molecules  and  forces.  The  teacher 
must  assist  him  here  and  must  bring  to  his  task  well- 
ordered  knowledge  of  these  things.  The  central  concep- 
tion must  be  the  transformation  of  energy  by  the  living 
tissues. 

•  Physiology  can  be  taught  to  the  best  purpose  to  pupils 
who  have  had  previous  courses  in  both  physics  and  chem- 
istry. In  the  introduction  to  this  book  it  is  pointed  out 
that  physiology  could  not  develop  historically  until  these 
sciences  were  well  advanced.  It  is  legitimate  to  argue 
that  it  cannot  be  grasped  by  the  individual  without  the 
elements  of  these  supporting  sciences  to  serve  as  its  foun- 
dation. The  next  best  thing  to  having  these  subjects 

11 


12     .  HUMAN    PHYSIOLOGY 

precede  physiology  is  to  have  them  simultaneous  with  it. 
If  only  one  can  be  taken  the  choice  must  fall  on  chemistry. 

The  teacher  who  is  called  upon  to  give  a  course  in 
physiology  to  students  who  have  had  neither -physics  nor 
chemistry  is  severely  handicapped.  They  must  soon  be 
using  phrases  which  cannot  mean  to  them  what  they  mean 
to  others  of  ampler  training.  They  may  recite  glibly 
and  to  the  satisfaction  of  an  easy-going  instructor,  but 
the  large  conceptions  may  be  wanting.  A  teacher  who 
has  to  make  the  most  of  such  a  situation  will  be  com- 
pelled to  sacrifice  much  in  other  directions  to  secure  clear- 
ness as  to  fundamentals.  Detail,  however  attractive, 
must  give  way  to  cardinal  truths. 

What  are  the  matters  which  must  be  impressed  at  any 
cost?  First  of  all,  the  conservation  of  energy  and  its 
convertibility  from  one  form  to  another.  Second,  the 
closely  related  fact  of  the  latency  of  energy  in  those  com- 
pounds which  we  call  fuels.  The  recognition  of  food  as  a 
biologic  fuel.  The  general  significance  of  oxidation  and 
the  release  of  potential  energy.  The  realization  that  the 
respiratory  process  is  a  particular  case  of  oxidation  and 
that  its  value  is  in  the  setting  free  of  energy  that  becomes 
manifest  as  heat  and  mechanical  work.  Finally,  the 
conception  that  the  development  and  application  of 
energy  are  determined  by  stimuli  brought  to  bear  upon 
organisms  from  the  world  without. 

Attention  may  be  called  to  one  of  the  many  sources  of 
confusion  which  troubles  beginners.  This  is  the  ques- 
tion of  scale.  The  student  has  thought  almost  altogether 
in  the  past  of  things  which  are  appreciated  by  the  naked 
eye.  When  he  enters  upon  his  study  of  physiology  he 
may  be  said  to  be  asked  to  conceive  of  three  orders  of 
magnitude.  The  features  of  gross  anatomy  present 
no  difficulty.  But  these  are  pictured  on  one  page  of  his 
book  while  on  the  next  there  may  be  a  representation  of 
cells.  The  teacher  must  be  at  pains  to  make  plain  what 
ratio  obtains  between  the  two. 

The  following  device  may  be  helpful.     A  fine  hair  may 


TO    THE    TEACHER  13 

have  a  diameter  of  J^0o  inch.  Six  red  corpuscles, 
lying  flat  in  a  row  with  their  edges  touching,  would 
about  span  the  cut  end  of  the  hair.  It  would  take  half 
a  dozen  bacteria  of  average  size,  laid  end  to  end,  to  reach 
across  the  disc  of  a  single  red  corpuscle.  When  the  micro- 
scope is  used,  it  is  a  convenience  to  bear  in  mind  the  real 
size  of  the  visible  field.  For  a  magnification  of  100  diame- 
ters this  is  usually  about  J^o  inch;  for  500  diameters 
it  is  about  J^ oo- 

At  the  same  time  that  the  pupil  is  required  to  think  in 
terms  of  microscopic  measurements  he  is  introduced  to 
molecular  and  atomic  ideas.  He  is  in  danger  of  failing 
to  realize  how  great  is  the  difference  in  scale  between  the 
two.  He  must  not  be  allowed  to  think  that  the  micro- 
scope can  bring  the  molecules  of  a  true  solution  to  visi- 
bility. The  effort  must  be  to  explain  to  him  that  this  is 
utterly  beyond  accomplishment.  The  step  from  the 
gross  structure  to  the  cellular  is  short  indeed  compared 
with  the  further  transition  to  the  realm  of  molecules. 

A  word  about  helps  in  teaching.  A  first-class  manikin 
is  a  treasure  greatly  to  be  desired.  Its  superiority  to 
charts  and  diagrams  lies  in  its  solidity,  its  representation 
of  three  dimensions  in  the  only  convincing  way  that  can 
be  employed.  However,  a  good  manikin  is  expensive. 
Those  which  are  flat  and  made  to  unfold,  layer  after 
layer,  are  not  realistic  enough  to  be  preferred  to  charts, 
and  charts  in  turn  may  be  dispensed  with  for  most  pur- 
poses if  the  teacher  will  cultivate  blackboard  drawing. 
The  great  advantage  of  chalk  lies  in  the  fact  that  what- 
ever feature  is  under  discussion  may  be  made  to  stand 
out  and  no  details  which  are  not  helpful  at  the  moment 
need  be  in  view.  It  is  much  easier  to  follow  the  exposi- 
tion when  the  structures  referred  to  appear  successively 
than  when  they  are  all  presented  at  once  as  in  a  com- 
pleted figure. 

The  teacher  of  physiology  must  take  a  middle  course  in 
the  endeavor  to  correct  two  extreme  tendencies  commonly 
manifested  by  immature  students.  The  very  same 


14  HUMAN    PHYSIOLOGY 

attitudes  have  been  illustrated  by  profound  thinkers.  A 
pupil  of  one  type  will  underrate  the  complexity  of  the 
problems  he  hears  about  and  will  become  a  mechanist  of 
the  crass  and  confident  sort.  Another  will  doubt  the 
value  of  all  attempts  to  penetrate  such  an  intricate  maze. 
To  the  temperate  mind  there  should  be  apparent  at  one 
and  the  same  time  the  profit  that  will  continually 
accrue  from  research  and  the  unlimited  extent  of  the 
work  still  to  be  done. 

The  last  chapter  of  this  book  has  the  character  of  an 
appendix  and  is  designed  more  for  the  teacher  than  for 
the  student. 


CHAPTER  I 
INTRODUCTION 

Physiology  and  Other  Sciences. — Physiology  has  been 
well  defined  as  "the  physics  and  chemistry  of  living 
matter."  If  this  is  a  fair  statement  it  is  a  peculiarly  ad- 
vanced and  difficult  sort  of  physics  and  chemistry,  for  it 
has  to  do  with  reactions  and  compounds  of  the  most  com- 
plex description.  It  could  not  be  developed  far  until 
the  physics  and  chemistry  of  non-living  matter  had  pre- 
pared the  way.  Another  science  also  had  to  come  before 
it,  namely,  anatomy,  the  study  of  the  structure  of  organ- 
isms. This  was  a  field  in  which  progress  could  be  made 
independently  of  other  discoveries  and  it  is  not  strange, 
therefore,  that  so  early  as  the  sixteenth  century  a  large 
mass  of  anatomic  knowledge  was  embodied  in  monu- 
mental books  with  finely  executed  plates  that  still  com- 
mand admiration.  The  principal  object  of  study  and 
delineation  was  the  human  rather  than  the  animal  organi- 
zation. In  the  next  century  anatomy  became  broadly 
comparative,  extending  to  many  forms,  and  at  the  same 
time  it  became  minute,  such  poor  microscopes  as  were 
available  being  diligently  employed. 

But  the  seventeenth  century  is  most  likely  to  be  asso- 
ciated in  our  thought  with  Descartes  and  Boyle,  Galileo 
and  Newton,  men  who  were  preeminently  physicists  or 
astronomers.  The  contemporary  chemistry  was  limited 
and  confused.  It  was  natural  that  the  great  addition  to 
physiologic  knowledge  made  in  this  period  should  have 
been  in  the  realm  of  physics.  This  was  the  conception 
of  this  circulation  of  the  blood.  The  arguments  in  sup- 
port of  the  doctrine  were  marshalled  by  William  Harvey 
in  1628  in  such  a  telling  fashion  that  it  was  soon  univer- 

15 


16  HUMAN    PHYSIOLOGY 

sally  accepted.  Just  about  a  hundred  years  later  the 
problems  of  the  circulation  were  skillfully  investigated 
from  a  quantitative  point  of  view  by  another  Englishman, 
Stephen  Hales.  In  general,  however,  physiology  in  the 
seventeenth  and  in  a  large  part  of  the  eighteenth  cen- 
tury consisted  mainly  of  inferences  drawn  from  anatomic 
structure.  Sometimes  these  were  exceedingly  shrewd 
while  others  were  recklessly  speculative. 

Late  in  the  eighteenth  century  the  science  of  chemistry 
was  rapidly  advanced  and  physiology  at  once  began  to 
profit  by  the  new  knowledge.  This  was  particularly 
true  in  the  investigation  of  respiration,  the  comparison 
between  living  organisms  and  other  agencies  of  oxida- 
tion, and  in  a  new  appreciation  of  the  nature  of  digestive 
processes.  It  was  at  this  time  that  a  fundamental  prin- 
ciple was  recognized  in  the  indestructibility  of  matter, 
the  teaching  that  substances  may  be  transformed  in 
many  ways  but  never  annihilated.  This  was  prophetic 
of  the  doctrine  of  the  conservation  of  energy  which  was 
to  be  established  seventy-five  years  later  and  which  has 
been  almost  as  influential  in  biology  as  in  physics. 

We  may  estimate  as  highly  as  possible  the  accumula- 
tion of  physiologic  facts  before  the  year  1800,  and  we 
shall  yet  feel  that  our  science  belongs  essentially  to  the 
nineteenth  century.  Many  of  its  cardinal  discoveries 
are  referred  to  dates  between  1840  and  1870.  The  mid- 
dle of  the  century  found  the  chemistry  of  organic 
compounds  well  developed  and  at  the  disposal  of  the 
physiologist.  Johannes  Miiller  (1801-1858)  has  been 
called  the  "Father  of  Modern  Physiology.'7  A  brilliant 
contributor  himself,  he  was  the  teacher  of  a  group  of 
distinguished  workers  who  diverged  into  various  fields 
to  multiply  observations  of  the  greatest  interest. 

Physiology  is  still  unfolding.  At  the  present  time 
there  are  many  laboratories  where  its  problems  are  under 
scrutiny  and  journals  in  several  languages  appear  each 
month.  The  literature  has  become  so  large  that  it  has 
to  be  compiled  and  presented  in  volumes  of  abstracts. 


INTRODUCTION  17 

Those  who  are  pursuing  researches  in  the  chemistry  of 
living  things  are  more  and  more  definitely  separated  from 
those  whose  studies  are  physical.  It  is  scarcely  possible 
to  be  an  authority  in  both  subdivisions.  Specialization 
constantly  becomes  more  pronounced.  For  example,  the 
subject  of  the  electric  phenomena  which  can  be  observed 
in  living  tissues  is  extensive  enough  by  itself  to  engage 
the  exclusive  attention  of  many  students. 

Methods  in  Physiology. — Something  must  now  be  said 
of  the  methods  which  physiologists  employ.  Evidently 
they  must  work  upon  living  matter  since  their  interest  is 
in  the  reactions  which  are  peculiar  to  it.  It  does  not 
follow  that  they  always  make  use  of  intact  organisms  for 
the  living  state  may  often  be  protracted  for  some  time 
in  portions  of  animals  (or  plants)  detached  from  the 
body.  Thus  the  muscles  of  a  frog's  leg  can  be  preserved 
for  hours  after  separation  from  the  other  systems  and 
will  behave  in  much  the  same  way  as  if  they  were  still 
united  with  them.  This  property  of  "survival,"  which 
is  so  valuable  to  experimenters,  is  much  more  to  be  relied 
on  in  the  so-called  cold-blooded  than  in  the  warm-blooded 
animals. 

For  carrying  out  many  researches  it  is  necessary  to  use 
living  animals.  We  must  frequently  mention  such  pro- 
cedures in  the  course  of  this  book  and  it  will  be  well  at 
the  outset  to  speak  briefly  of  vivisection  and  the  objec- 
tions which  are  raised  against  it.  It  is  not  strange  that 
experiments  on  animals  should  be  viewed  with  abhorrence 
by  those  who  have  been  influenced  by  the  highly  colored 
accounts  of  scientific  misdoing  which  are  so  widely  cur- 
rent. The  feeling  of  compassion  and  the  impulse  to 
protect  all  creatures  from  suffering  are  so  admirable  that 
the  physiologist  must  have  a  certain  sympathy  with  his 
most  violent  critic.  Nevertheless  he  feels  that  the  op- 
position to  his  methods  is  due  mainly  to  ignorance  of 
actual  conditions  and  misapprehension  of  the  spirit  of 
the  investigator. 

A  prime  fact  to  be  reckoned  with  is  that  physiologists 


18  HUMAN    PHYSIOLOGY 

are  as  humane  as  other  educated  men  and  as  reluctant 
to  inflict  pain.  The  contrary  assertion  is  familiar,  but 
it  may  be  questioned  whether  it  has  ever  been  made  by 
anyone  enjoying  a  wide  acquaintance  among  such  work- 
ers. In  the  second  place,  the  physiologist  can  almost 
always  avoid  giving  pain  and  actually  promotes  the 
success  of  his  work  by  excluding  it.  Pain  would  be  a 
disturbing  factor  in  most  experiments  and  the  most 
cold-blooded  scientist  would  have  reason  to  prevent  it. 
Many  years  ago,  when  there  were  no  anesthetics  to  be 
used,  animals  were  certainly  made  to  suffer  severely 
that  physiologic  knowledge  might  be  advanced.  The 
results  of  the  early  work  have  proved  so  valuable  that 
we  are  glad  that  it  was  done;  at  the  same  time  we  con- 
gratulate ourselves  that  the  work  can  be  continued  with- 
out the  infliction  of  pain. 

It  is  sometimes  urged  that  we  have  no  right  to  deprive 
animals  of  life  or  liberty  for  scientific  ends.  This  cannot 
well  be  maintained  with  any  show  of  consistency  unless 
the  advocate  is  a  vegetarian.  We  can  do  without  meat 
and  it  will  hardly  be  claimed  that  we  are  justified  in 
destroying  animals  for  the  gratification  of  appetite  and 
not  for  the  increase  of  knowledge.  Most  humanitarians 
recognize  the  necessity  of  doing  away  with  superfluous 
animals  and  make  this  a  prominent  function  of  their 
organizations.  The  subjective  experience  of  a  dog  or 
cat  killed  by  ether  or  chloroform  could  not  be  different 
if  an  operation  were  performed  upon  it  before  it  died 
from  what  it  would  be  if  the  anesthetic  were  at  once 
forced  to  a  fatal  intensity. 

In  exceptional  cases  animals  are  allowed  to  recover 
from  anesthesia  and  are  kept  alive  to  observe  the  later 
effects  of  an  operation.  This  may  sometimes  involve 
suffering,  but  it  may  also  be  the  only  way  to  discover 
important  truths.  It  is  a  fact  that  such  experiments  are 
performed  with  genuine  reluctance  by  the  typical  physiolo- 
gist and  in  a  spirit  like  that  which  animates  any  other 
surgeon.  Whatever  hardship  may  have  been  imposed 


INTRODUCTION  19 

upon  animals  in  our  laboratories  cannot  for  a  moment 
be  compared  with  the  thoughtless  cruelty  of  hunters  and, 
in  the  writer's  opinion,  with  the  abuse  of  pets  by  children. 
%  Vivisection  has  been  effectively  defended  on  the  ground 
that  it  has  been  indispensable  to  medical  progress.  This 
argument  will  not  be  extended  here  but  the  question 
asked  by  a  great  surgeon — W.  W.  Keen — may  be  repeated 
for  the  consideration  of  the  reader:  " Reckoned  in  rab- 
bits, what  is  the  value  of  your  wife,  your  husband,  or 
your  child?" 

The  Mechanistic  Conception. — Experimenters  of  the 
present  time  believe  that  the  organisms  with  which  they 
have  to  do  are  mechanisms  in  the  sense  that  none  of  the 
recognized  principles  of  physics  and  chemistry  are  vio- 
lated in  their  working.  This  conception  is  held  in  place 
of  an  earlier  one,  now  spoken  of  as  that  of  the  Vitalists, 
according  to  which  living  things  were  regarded  as  unique 
in  character  and  not  bound  by  all  the  limitations  of 
strict  mechanisms.  Of  course  it  is  recognized  that  there 
is  an  abundance  of  mystery  about  life  and  its  manifesta- 
tions, but  all  progress  toward  a  better  understanding  of 
plants  and  animals  has  thus  far  been  based  on  the  view 
that  they  exemplify  the  same  laws  which  are  accepted 
as  applicable  to  things  not  living.  While  this  is  true  it 
is  probably  correct  to  say  that  the  physiologist  of  to-day 
has  a  more  adequate  realization  of  the  complexity  of  his 
problems  than  his  predecessor  had  a  generation  ago.  It 
must  be  borne  in  mind  that,  except  for  certain  passages 
in  the  treatment  of  the  brain,  the  point  of  view  is  objec- 
tive, that  is,  the  concern  is  with  what  happens  in  a  mate- 
rial body  and  not  with  what  is  felt  while  the  various 
actions  are  going  on. 

Brief  reference  has  been  made  to  the  principles  of  the 
indestructibility  of  matter  and  the  conservation  of 
energy.  These  were  adopted  by  men  of  science  as  the 
result  of  work  in  the  fields  of  chemistry  and  physics. 
But  we  have  every  reason  to  believe  that  they  hold  good 
for  living  forms  of  every  grade,  human  beings  included. 


20  HUMAN    PHYSIOLOGY 

Animals  have  an  income  and  an  outgo  of  matter  and  the 
two  are  strictly  balanced  unless  the  body  is  growing  or 
wasting.  They  have,  similarly,  an  income  and  an  outgo 
of  energy  and  the  proof  that  here,  too,  nothing  is  lost  or 
gained  stands  forth  as  one  of  the  foremost  achievements 
of  our  science  in  the  last  century.  It  would  probably 
not  have  surprised  a  vitalist  of  the  old  school  if  it  had 
been  shown  that  an  animal  could  develop  energy  inde- 
pendently of  any  external  supply.  This  would  have 
made  it  a  creator  or  generator  while  we  have  come  to  regard 
it  as  a  transformer. 

Adaptation. — Biologists  have  made  many  efforts  to  de- 
fine life.  Their  attempts  have  often  been  ingenious, 
though  never  entirely  satisfying.  Ignoring  the  possi- 
bility of  consciousness  in  the  organism  observed  (though 
necessarily  recognizing  its  existence  for  the  observer)  a 
great  thinker  has  said  that  life  is  "the  continual  adjust- 
ment of  internal  to  external  relations."  It  is  certainly 
true  that  we  judge  by  this  standard  when  we  attempt  to 
decide  whether  a  mass  of  matter  is  living  or  dead.  If  it 
shows  no  tendency  to  protect  itself  from  the  changing 
conditions  which  are  brought  to  bear  upon  it  we  conclude 
that  it  is  lifeless.  This  is  what  we  should  infer  of  a  dog 
which  lay  in  the  road  indifferent  to  the  approach  of  an 
automobile.  We  look  to  see  organisms  adapting  them- 
selves to  their  circumstances  so  as  to  preserve  their 
integrity  in  spite  of  many  hostile  forces.  The  study  of 
adaptation  is,  accordingly,  a  large  part  of  physiology. 

The  hard  thing  for  the  beginner  is  to  regard  this  famil- 
iar adaptation  as  a  mechanical  process.  He  has  always 
thought  that  the  quest  of  food,  the  avoidance  of  enemies, 
the  protection  of  the  body  against  heat  and  cold  were 
dictated  by  intelligence.  Introspection  encourages  such 
a  view  by  emphasizing  the  strength  of  the  desire  to  pro- 
long one's  life.  At  the  same  time  reflection  convinces 
one  that  it  is  impossible  to  give  the  attention  to  all  the 
serviceable  reactions  which  are  constantly  taking  place. 
Many  of  these  usually  pass  unnoticed  while  in  others 


INTRODUCTION  21 

undue  attention  is  more  apt  to  result  in  a  bungling  than 
a  superior  performance.  We  have  been  led  to  the  belief 
that  they  are  the  inevitable  result  of  structure  and  not 
of  present  intelligence. 

When  we  are  overheated  we  perspire  and  the  evapora- 
tion of  water  cools  the  skin  and  the  fraction  of  the  blood 
which  is  flowing  through  it.  This  is  an  adaptive  change 
but  it  is  obviously  one  which  we  can  scarcely  influence 
"by  taking  thought."  It  is  like  the  adjustment  which 
is  made  by  the  pendulum  of  a  clock  when  the  temperature 
rises.  By  an  ingenious  arrangement  of  different  metals 
the  tendency  of  the  pendulum  to  lengthen,  and  so  to  be 
slowed,  is  offset.  The  preservation  of  the  living  organ- 
ism through  the  summer  day  and  the  protection  of  the 
clock  against  the  same  disturbing  agency  are  both  ex- 
amples of  adjustment  due  to  structural  characters.  It 
may  be  urged  that  the  capacity  of  the  clock  to  regulate 
its  action  under  these  conditions  is  owing  to  the  wisdom 
and  foresight  of  its  maker  and  a  reverent,  parallel  infer- 
ence has  often  been  drawn  for  the  living  organism. 

The  adaptive  changes  which  an  animal  must  execute  to 
meet  emergencies  great  and  small  are  evidently  varied 
from  moment  to  moment  and  have  no  fixed  order  or  suc- 
cession. There  are  other  life  processes  which  are  more 
constant  and  monotonous.  These  can  generally  be  said 
to  be  related  to  maintenance.  The  mechanisms  of  breath- 
ing and  the  circulation,  of  alimentation  and  excretion, 
may  be  held  to  serve  primarily  for  the  maintenance  of  the 
organism  as  a  whole,  though  the  property  of  adaptability 
is  frequently  illustrated  in  connection  with  them.  The 
adaptive  mechanisms  par  excellence  are  the  muscles  and 
the  sense-organs  together  with  the  central  nervous  system 
which  correlates  the  former  with  the  latter.  In  recent 
writings  the  sense-organs  are  often  called  the  receptors 
while  the  muscles  and  the  glands  which  are  played  upon 
through  the  central  nervous  system  are  named  effectors. 

The  two  names  just  used  should  explain  themselves. 
A  receptor  is  a  structure  which  is  exposed  to  external 


HUMAN    PHYSIOLOGY 

influences — the  eye  to  light,  the  nerve-endings  in  the  skin 
to  pressure,  etc. — an  effector  is  a  structure  capable  of  re- 
action in  some  measurable  fashion.  The  possible  responses 
of  effectors  include  movement  or  its  suspension,  secretion 
or  its  suppression,  and  more  rarely  other  phenomena 
such  as  the  electric  discharge  of  the  torpedo  or  the  flash 
of  the  fire-fly.  Care  was  taken  to  include  in  the  foregoing 
statement  the  negative  types  of  reaction  which  should 
never  be  ignored.  The  word  inhibition  is  used  to  mean  a 
suspension  of  activity  of  effectors  brought  about  by  way 
of  the  nerves.  We  shall  find  it  a  more  important  ele- 
ment in  our  analysis  than  might  be  supposed. 

Coordination. — A  very  little  consideration  of  one  of  the 
higher  animals  convinces  us  that,  while  there  are  many 
parts  or  organs  at  work,  the  whole  creature  is  more  than 
an  aggregate,  it  is  an  individual.  To  say  this  is  to  imply 
that  in  some  way  all  the  parts  interact  for  the  common 
good.  We  say  that  their  activities  are  coordinated. 
Another  term  has  come  to  be  used  to  express  the  same 
fact :  we  say  that  the  local  actions  are  integrated  by  various 
means.  To  integrate,  or  confer  integrity,  is  to  bring  the 
associated  systems  into  such  relationship  that  wholeness, 
unity,  or  individuality  may  characterize  their  union.  A 
profound  lesson  may  be  apprehended  in  Kipling's  story 
of  "The  Ship  that  Found  Herself."  The  new  freighter 
went  out  from  a  British  port  upon  her  first  voyage. 
She  encountered  gales  and  head-seas  which  put  her 
structure  and  equipment  to  the  severest  test.  As  she 
was  racked  in  the  storms  every  frame  and  plate,  every 
bolt  and  rivet,  raised  a  separate  voice  of  complaint  and 
recrimination  against  its  mates.  The  discordant  clamor 
lasted  through  the  tedious  trip.  At  length,  on  the  fine 
morning  when  the  ship  glided  into  the  harbor  of  New 
York,  there  was  a  moment  of  silence  and  then  a  great 
voice  not  heard  before,  the  voice  of  the  "Dimbula" 
herself,  took  the  place  of  all  the  jarring  tones.  The  ship 
was  from  that  hour  an  organism  and  not  merely  an 
aggregation  of  parts. 


INTRODUCTION  23 

As  it  docs  not  call  for  much  imagination  to  think  of  a 
ship  or  a  locomotive  as  an  integrated  being,  it  should  be 
still  easier  to  recognize  this  essential  fact  in  the  world  of 
living  things.  Someone  has  said  that  in  the  case  of  an 
animal  "the  whole  is  greater  than  the  sum  of  its  parts" 
and  this  is  true  in  almost  the  same  sense  that  a  number 
made  by  combining  three  or  four  figures  is  greater  than 
the  sum  of  these  digits.  The  interaction  of  parts  makes 
possible  results  which  could  not  be  attained  by  the  parts 
while  isolated. 

The  Means  of  Coordination. — When  one  tries  to 
decide  how  the  evident  condition  of  coordination  or  inte- 
gration is  secured,  one  is  likely  to  think  first  of  the  nerv- 
ous system  as  serving  just  this  purpose.  This  is  a  correct 
idea,  provided  only  that  one  does  not  fail  to  reserve  a 
place  for  other  agencies.  When  rapid  changes  in  a  cer- 
tain region  promptly  follow  changes  somewhere  else  it 
can  usually  be  inferred  that  the  connecting  link  has  been 
a  nervous  one.  When,  for  example,  a  blow  has  fallen 
upon  the  head  and  the  arm  is  thrown  up  to  ward  off  an- 
other, the  case  is  one  of  coordination  through  the  medium 
of  the  nervous  system.  But  the  more  gradual  modifying 
of  the  activities  of  one  organ  in  consequence  of  those  of 
another  has  often  a  different  basis.  It  may  be  due  to  the 
passage  of  chemical  products  from  the  place  of  their  ori- 
gin to  some  other  locality  where  they  can  exert  an  influ- 
ence. The  importance  of  the  chemical  factors  in  the 
regulation  of  organic  processes  is  more  appreciated  to-day 
than  ever  before  and  it  is  likely  to  have  a  still  larger  recog- 
nition in  the  future. 

An  illustration  may  be  given.  In  the  normal  course  of 
digestion  the  pancreas  is  found  to  enter  upon  the  task 
of  secreting  its  valuable  juice  at  about  the  time  when  the 
stomach  begins  to  transfer  its  contents  to  the  small  in- 
testine. This  is  just  when  the  pancreatic  secretion  is 
needed  and  the  timeliness  of  the  action  makes  us  curious 
to  know  how  it  is  brought  about.  The  communication 
between  the  stomach  and  the  pancreas  has  been  found 


24  HUMAN    PHYSIOLOGY 

to  be  less  by  means  of  the  nervous  system  than  by  the 
passage  of  chemical  substances  through  the  circulation. 
Such  chemical  messengers,  arising  in  certain  parts  of  the 
body  and  perhaps  affecting  very  remote  parts,  are  com- 
monly referred  to  as  hormones.  The  very  slow  processes 
of  growth  and  development  are  known  to  be  greatly 
dependent  upon  the  interchange  of  hormones. 

Stimuli. — If  objective  life  consists  in  the  adjustment  of 
internal  to  external  conditions  we  shall  do  well  to  con- 
sider somewhat  more  fully  than  we  have  done  the  nature 
of  the  external  factors.  Any  external  condition  which 
modifies  the  activities  of  a  living  organism  may  be  called 
a  stimulus.  One  naturally  thinks  of  contacts,  changes  of 
temperature,  chemical  applications,  and  electric  shocks. 
Light  must  not  be  left  out  of  our  list.  A  stimulus  is  best 
thought  of  as  a  change  rather  than  a  continued  environ- 
mental state.  A  moment's  thought  will  help  one  to  real- 
ize that  a  sustained  condition  may  favor  the  preservation 
of  the  plant  or  the  animal,  but  it  is  the  shifting  of  outward 
conditions  which  cause  it  to  exhibit  its  capacity  for  reac- 
tion. Generally  speaking,  the  more  suddenly  a  change 
occurs  the  more  marked  is  its  effect  upon  living  matter. 

It  would  not  be  quite  accurate  to  describe  a  stimulus  as 
a  force.  A  force  may  stimulate  but  so  may  the  discon- 
tinuance of  a  force  which  has  been  operative  for  some  time. 
Positive  effects  from  negative  factors  are  common  enough. 
Silence  may  constitute  a  stimulus  when  it  succeeds  an 
accustomed  sound.  (Note,  for  instance,  how  one  wakes 
at  sea  when  the  measured  throb  of  the  engine  is  inter- 
rupted.) We  cannot  deny  that  the  reduction  of  the  tem- 
perature of  the  skin  is  a  source  of  stimulation,  though  this 
is  a  subtraction  of  energy  from  the  tissues  rather  than  a 
contribution. 


CHAPTER  II 
PLANTS  AND  ANIMALS 

While  scientists  believe  implicitly  in  the  conservation 
of  energy  they  recognize  also  the  principle  which  is  some- 
times spoken  of  as  "the  degradation  of  energy."  Ac- 
cording to  this  teaching,  though  the  total  quantity  of 
energy  in  the  universe  cannot  grow-  less  it  can  be  indefi- 
nitely dissipated.  Thus  our  sun  and  the  planets  in 
its  system  are  losing  heat  into  the  depths  of  measure- 
less space.  The  amount  of  energy  in  the  earth  is  not  a 
constant,  but  a  diminishing  store.  Since  life  as  we  now 
know  it  is  dependent  upon  the  maintenance  of  a  certain 
temperature  it  cannot  continue  forever  in  a  cooling 
environment. 

Oxidation. — Our  world  has  an  internal  store  of  heat, 
but  this  does  not  suffice  to  make  its  surface  the  abode  of 
many  forms  of  life  save  as  it  is  supplemented  by  the  rays 
of  the  sun.  Wherever  these  fall  daily  from  a  considerable 
elevation  above  the  horizon  the  temperature  favors  living 
organisms.  But  the  service  of  the  sun  to  the  earth  is 
not  limited  to  the  retaining  of  its  surface  at  a  desirable 
temperature  level.  The  radiant  energy  is  applied  to  the 
formation  of  what  we  call  the  organic  compounds  and  while 
these  exist  it  is  held  latent  in  them  awaiting  release. 

A  very  crude  classification  of  the  substances  we  find  in 
nature  might  be  attempted  by  separating  those  which  will 
burn  from  those  that  will  not.  Many  things  will  burn  at 
high  temperatures  which  are  not  ordinarily  thought  of  as 
combustible.  For  example,  this  is  true  of  iron.  If  we 
restrict  ourselves  to  the  consideration  of  those  things 
which  we  regard  as  fuels  and  which  burn  easily  we  shall 
be  struck  with  the  fact  that  they  are  products  of  past  life, 

25 


26  HUMAN    PHYSIOLOGY 

in  other  words,  they  are  organic  in  character.  Wood, 
animal  grease,  vegetable  oils,  alcohol — these  have  but 
recently  come  from  the  sphere  of  living  organisms. 
Coal,  petroleum,  and  natural  gas  have  arisen  in  connec- 
tion with  pre-historic  life.  We  say  of  such  materials  that 
they  are  energetic  or  that  they  have  distinct  fuel-values. 

The  burning  of  fuels  is  a  chemical  process  in  which  oxy- 
gen, usually  supplied  from  the  atmosphere,  unites  with 
their  principal  elements.  Two  of  these  elements,  carbon 
and  hydrogen,  have  particularly  to  be  considered.  When 
oxygen  has  united  with  carbon  to  the  full  extent  that  it 
will  do  so  a  gaseous  product,  carbon  dioxid,  is  formed. 
This  is  also  known  as  carbonic  acid  gas;  it  is  familiar  in 
the  bubbles  of  soda  water  and  it  is  the  chief  gas  going 
up  the  chimney  from  a  coal  fire.  When  oxygen  combines 
with  hydrogen,  water  is  formed  and  much  more  water 
arises  from  combustion  than  is  commonly  realized.  In 
the  flame  of  a  candle  oxygen  is  uniting  with  the  carbon 
and  hydrogen  of  the  wax  to  produce  carbon  dioxid  and 
water  vapor.  The  ascending  column  of  hot  gases  above 
the  flame  is  composed  very  largely  of  these  two  compounds. 

The  generation  of  carbon  dioxid  and  water  by  the  lit- 
eral burning  of  organic  substances  is  not  the  only  mode  of 
their  formation.  Similar  materials  are  constantly  under- 
going the  same  resolution  in  a  more  gradual  way  and 
without  the  accompaniment  of  flame  and  smoke.  If  we 
were  to  add  " without  heat"  we  should  be  wrong  for  it 
has  been  shown  that  just  as  much  heat  is  evolved  in  the 
slowest  as  in  the  quickest  oxidation  of  a  given  compound. 
Of  course  the  heat  production  will 'be  much  less  obvious 
if  it  is  extended  over  a  long  time. 

Physiological  Oxidation. — The  most  interesting  cases 
of  oxidation  of  the  more  gradual  type  are  those  which  go 
on  under  the  influence  of  living  matter  and,  indeed,  con- 
stitute the  most  conspicuous  part  of  its  activity.  Where 
there  is  life  there  will  usually  be  perceptible  oxidation. 
In  the  plant,  as  in  the  candle,  organic  matter  is  continu- 
ally being  degraded  and  again,  carbon  dioxid  and  water 


PLANTS    AND    ANIMALS  27 

are  the  major  products  set  free.  This  may  be  said  with 
equal  truth  of  the  animal  and  it  may  be  pointed  out  that 
if  the  candle  is  an  old-fashioned  one,  made  of  tallow,  the 
fuel  is  the  same  that  might  have  been  used  by  the  animal 
if  its  life  had  not  been  cut  short. 

When  we  deliberately  cause  fuels  to  burn,  our  object  is 
usually  to  make  some  use  of  the  energy  which  has  been 
latent  in  them.  We  light  the  candle  or  the  lamp  that  it 
may  give  us  light.  Fires  are  maintained  to  warm  our 
houses,  to  bring  about  desirable  changes  in  our  food, 
or  to  keep  machinery  in  motion.  In  every  case  the 
object  is  secured  through  the  release  of  stored  energy. 
It  has  been  potential  before  and  it  now  becomes  kinetic 
or  active.  The  service  of  oxidation  to  the  animal  is  not 
essentially  different.  It  is  the  source  of  animal  heat 
and  muscular  power.  It  is  hard  for  the  elementary 
student  to  grasp  the  truth  that  decomposition  is  not  dis- 
aster, but  a  necessary  condition  of  living.  It  is  only  by 
expenditure,  by  the  sacrifice  of  resources,  that  the  organ- 
ism can  react  and  prove  itself  alive.  The  stores  of  the 
body  are  like  money,  useful  not  in  themselves,  but  be- 
cause of  the  results  obtained  in  exchange  for  them. 

If  it  were  possible  to  suspend  oxidation  in  the  body  of 
one  of  the  higher  animals  it  might  be  thought  that  it 
would  remain  motionless  and  cold  but  perfectly  preserved 
for  an  indefinite  time.  Such  a  suspension  of  animation  is 
unknown  among  higher  forms;  in  thenAhe  restriction  of 
oxygen  supply  perverts  the  life  processes  before  it  stops 
them  and  the  result  is  the  poisoning  which  we  call  as- 
phyxia. Lower  down  in  the  scale  we  find  such  modifi- 
cations of  living  matter  as  the  seeds  of  plants,  the  spores 
of  bacteria,  and  the  encysted  forms  of  certain  aquatic 
animals  which  do  represent,  approximately  at  least,  an 
arrest  of  respiration,  and  so  of  activity,  which  may  be 
continued  for  a  long  while.  A  German  writer  has  com- 
pared the  state  of  such  forms  with  that  of  a  clock  which 
is  wound  up  but  not  going.  There  is  the  capacity  for 
action  but  the  mechanism  remains  under  restraint. 


28  HUMAN    PHYSIOLOGY 

Photosynthesis. — If  the  activities  of  living  things  all 
depend  upon  the  oxidation  of  organic  matter,  how  is 
the  supply  kept  up?  This  is  one  of  the  great  questions 
which  biologists  have  been  called  upon  to  answer  and 
they  have  been  able  to  throw  much  light  upon  it.  It 
has  already  been  hinted  that  the  energy  of  the  sun 
enters  into  our  reckoning.  It  is  this  energy  which  is 
applied  to  the  reconstruction  of  fuels  from  the  simple 
products  of  their  decomposition.  A  work  like  this  is 
manifestly  the  reverse,  in  a  chemical  sense,  of  oxidation 
and  can  be  spoken  of  as  reduction.  It  is,  at  the  same 
time  a  synthesis  and,  since  it  occurs  under  the  driving 
power  of  radiant  energy,  a  photosynthesis. 

Photosynthesis  is  accomplished  chiefly  by  the  higher, 
and  pigmented  plants.  To  say  green  plants  would  be 
nearly  but  not  wholly  correct.  Every  green  leaf  upon 
which  the  rays  of  the  sun  are  falling  is  capable  of  making 
starch  and  other  energetic  compounds  from  the  carbon 
dioxid  and  water  vapor  which  are  obtainable  from  the  air. 
Since  it  is  the  reversal  of  combustion  or  oxidation  it  fol- 
lows that  when  it  is  proceeding  oxygen  must  be  set  free. 
It  is  interesting  to  recall  the  occasion  upon  which  this 
important  fact  was  first  demonstrated.  Joseph  Priestley, 
an  English  scholar  whose  life  story  is  peculiarly  absorb- 
ing, prepared  oxygen  gas  and  observed  its  relation  to 
combustion  in  1772.  He  found  that  in  a  confined  vol- 
ume of  air  only  a  limited  quantity  of  inflammable  matter 
could  be  burned.  But  he  soon  made  the  momentous 
discovery  that  the  power  to  support  combustion  could  be 
restored  to  the  exhausted  air  in  a  jar  by  allowing  a  green 
plant  to  grow  inside  it. 

It  is  owing  to  the  existence  of  colored  plants  that  the 
composition  of  the  atmosphere  changes  but  very  slightly 
from  century  to  century.  They  abstract  from  it  the 
carbon  dioxid  which  has  come  from  all  the  fires  in  the 
world,  from  all  animal  life,  and  from  the  vegetable  king- 
dom too,  for  photosynthesis  does  not  take  the  place  of 
respiration  in  plants;  it  is  a  process  which  accompanies 


PLANTS    AND    ANIMALS  29 

respiration  and  goes  on  only  under  favoring  conditions. 
At  night  all  living  things  are  consuming  oxygen  and  giv- 
ing forth  carbon  dioxid.  The  same  reactions  occur  by 
day  but  the  constructive  activities  of  the  higher  plants 
are  then  so  remarkable  that  we  are  prone  to  forget  the 
undercurrent  which  is  still  setting  in  the  opposite  direc- 
tion. Sometimes  it  is  said  that  carbon  dioxid  and  water 
are  the  principal  foods  of  green  plants,  but  it  is  more  ac- 
curate to  say  that  such  plants  have  the  power  to  make  their 
foods  from  simple  raw  materials. 

Attention  should  be  called  to  the  fact  that  light  is  not, 
on  the  whole,  a  factor  that  is  favorable  to  life.  It  is 
highly  destructive  to  the  lower  organisms  and  its  value 
to  man  is  indirect.  It  is  helpful  to  him  because  it  de- 
stroys some  of  his  enemies,  because  it  keeps  up  the  food 
supply  of  the  world,  and  because  of  its  relation  to  his 
intellectual  interests.  Light  is  probably  always  detri- 
mental rather  than  beneficial  to  defenseless  living  matter. 
This  is  to  say  that  all  transparent  organisms  are  subject 
to  injury  through  its  influence.  An  important  function 
of  the  pigment  (chlorophyll)  in  those  plants  which  profit 
by  the  light  is  probably  to  turn  back  from  the  leaves  most 
of  its  searing  rays  and  to  admit  only  selected  ones  to  the 
laboratories  where  the  photosynthesis  goes  on.  Our 
experiences  with  sunburn  remind  us  that  light  may  harm 
the  human  skin,  while  the  development  of  tan  suggests  a 
protective  reaction.  It  is  not  unreasonable  to  say  that 
the  chief  reason  why  we  are  not  more  damaged  by  light 
is  that  we  are  too  thick;  it  does  not  pierce  to  the  seat  of  our 
vital  processes. 

Recognizing  this,  we  find  the  synthetic  application  of 
light  all  the  more  remarkable  and  the  scale  on  which  the 
action  proceeds  is  too  vast  for  the  imagination.  The 
annual  harvest  of  all  the  nations  and  the  cut  of  timber 
give  us  an  inkling  of  it.  These  immense  returns  from  the 
vegetable  world,  we  must  remember,  are  produced  only 
to  a  small  extent  from  the  soil  but  very  largely  from  the 
air.  This  was  the  fact  which  surprised  Van  Helmont  as 


30  HUMAN    PHYSIOLOGY 

he  reviewed  a  certain  memorable  experiment  made  about 
three  hundred  years  ago.  He  had  planted  a  young  shoot 
of  willow  in  a  tub  of  earth  and  let  it  grow  for  five  years. 
It  had  then  increased  in  weight  by  about  160  pounds, 
though  the  soil  had  lost  only  a  few  ounces.  So  far  as 
Van  Helmont  could  judge  the  tree  must  have  been  made 
from  the  water  that  had  been  freely  supplied.  Water 
had  indeed  entered  into  its  structure,  but  a  larger  addi- 
tion was  due  to  the  unrecognized  carbon  dioxid  of  the 
atmosphere. 

Interrelations  of  Plants  and  Animals. — It  is  now  evi- 
dent that  a  certain  reciprocity  exists  between  plants  and 
animals.  But  this  is  true  only  when  the  plants  consid- 
ered are  those  with  pigment;  the  uncolored  varieties — 
fungi,  etc. — are  unable  to  make  the  fuels  which  they  con- 
sume and  are,  therefore,  like  the  animals  in  their  depend- 
ence on  the  higher  plants.  Some  forms  exist  which  have 
intermediate  powers.  It  is  to  be  borne  in  mind,  that, 
while  the  differences  between  typical  animals  and  plants 
such  as  we  naturally  choose  for  comparison  are  striking 
enough,  there  are  numerous  species  low  down  in  the  scale 
which  are  not  surely  to  be  assigned  to  one  class  or  the 
other.  Some  of  these  have  been  claimed  alternately  by 
the  botanists  and  the  zoologists. 

Excluding  all  but  the  most  highly  developed  types,  let 
us  consider  how  the  proximity  of  plants  and  animals  in- 
fluences the  economy  of  the  two.  A  specific  illustration 
may  be  suggested.  Suppose  that  a  snail  is  living  in  a  hot- 
bed. The  animal  eats  portions  of  the  plants,  living  or 
dead.  It  returns  carbon  dioxid  and  water  to  the  air 
and  soil  of  the  enclosure.  The  green  leaves,  transmuting 
the  energy  of  light  rays,  can  recover  the  fuel  which  the 
animal  has  dissipated.  In  thus  compensating  for  the 
spendthrift  proclivities  of  the  animal  the  plants  have 
returned  to  the  air  the  oxygen  which  the  animal  appropri- 
ated. It  is  clear  that  the  plants  are  necessary  to  the 
animal  and  that  they  must  grow  fast  enough  to  make  up 


PLANTS    AND    ANIMALS  31 

for  the  foraging  of  the  snail  or  be  exterminated.  If  they 
are  destroyed  the  mollusc  will  be  left  to  starve. 

Is  the  animal  necessary  to  the  plants?  The  gardener 
will  say  that  it  is  not  and  in  the  actual  conditions  of 
nature  the  snail  can  well  be  spared.  There  are  other 
sources  of  carbon  dioxid  than  the  respiration  of  animals, 
and  water  is  abundant  enough  in  the  world.  The  soil 
of  the  hotbed  doubtless  swarms  with  lowly  and  colorless 
plants  which  are  oxidizing  the  surplus  organic  matter 
much  as  the  animal  does.  Moreover,  these  organisms  of 
the  soil,  mainly  bacteria,  affect  usually  the  dead  fragments 
detached  from  the  green  plants  above  and  do  not  prey 
upon  their  living  leaves  as  the  animal  does  so  ruthlessly. 

Nitrogenous  Compounds. — So  far  we  have  limited  our 
discussion  to  the  formation  of  fuels  like  starch  or  oils 
which  yield  no  other  products  than  carbon  dioxid  and 
water  when  they  are  completely  oxidized.  Such  com- 
pounds are  the  chief  source  of  energy  for  both  plants  and 
animals.  But  where  life  is  manifested  there  will  always 
be  chemical  compounds  of  another  sort  and,  indeed,  the 
bodies  which  are  now  to  be  discussed  seem  to  be  more  in- 
timately connected  with  life  itself  than  the  standard 
fuels  we.  have  been  considering.  The  compounds  in 
question  are  the  proteins. 

Proteins  contain  nitrogen.  In  a  much  smaller  percent- 
age they  also  contain  sulphur.  Phosphorus  is  present  in 
some  but  not  in  the  majority  of  the  proteins.  It  will  be 
plain  that  such  compounds  cannot  be  oxidized  to  carbon 
dioxid  and  water  exclusively  for  the  nitrogen  and  the 
sulphur  must  be  represented  in  some  form  among  the  de- 
composition products.  Neither  can  we  have  any  forma- 
tion of  proteins  without  a  supply  of  nitrogen  and  sulphur. 
At  another  time  we  may  have  occasion  to  emphasize  the 
fact  that  the  distinguishing  character  of  the  proteins  is 
not  so  much  the  list  of  the  elements  which  enter  into  them 
as  the  complex  fashion  in  which  these  elements  are  com- 
bined. For  the  present  we  are  concerned  rather  with  the 
exchanges  of  these  elements  which  occur  in  nature. 


32  HUMAN    PHYSIOLOGY 

Plants  synthesize  proteins  as  well  as  starch.  Little  is 
known  of  the  process  in  detail,  but  experience  of  the  most 
practical  kind  shows  that  the  most  available  supply  of 
nitrogen  for  use  in  protein  formation  is  afforded  by  the 
nitrates  of  the  soil.  These  salts  are  received  into  the  roots 
of  plants  and  are  transported  in  the  sap.  The  sulphur 
required,  a  smaller  quantity  than  the  nitrogen,  is  secured 
in  a  similar  way  as  dissolved  sulphates.  So  phosphorus 
comes  into  the  plant  in  the  form  of  phosphates.  These 
various  salts  we  aim  to  furnish  when  we  fertilize  a  plot  of 
ground. 

Animals  take  their  proteins  ready-made  from  plants. 
They  do  not,  however,  store  the  identical  proteins  which 
they  have  eaten.  They  digest  them — a  process  of  decom- 
position— and  they  synthesize  a  selected  fraction  of  the 
products.  But  this  is  a  much  less  radical  reconstruction 
than  that  performed  by  the  plants  in  manufacturing  pro- 
teins from  the  simplest  materials.  The  earlier  writers 
used  to  make  the  sweeping  assertion  that  animals  are 
altogether  destructive  and  can  never  carry  on  any  con- 
structive work.  This  is  now  seen  to  be  untrue ;  destruc- 
tive changes  predominate  in  the  sum  of  their  life  activities, 
but  they  often  make  complex  compounds  from  simpler 
ones  in  special  cases. 

It  may  be  asked  how  animals  can  synthesize  chemical 
compounds  of  high  fuel-value  when  they  cannot  apply  to 
the  task  the  energy  of  light.  The  answer  is  found  in  the 
fact  that  where  the  prevailing  reactions  are  in  the  line  of 
oxidation  and  attended  by  a  release  of  abundant  energy 
a  portion  of  this  energy  may  be  employed  to  advance 
changes  of  the  opposite  order,  those  in  which  heat  or  other 
energy  is  absorbed.  A  man  on  first  observing  the  hydrau- 
lic ram  at  work  may  be  surprised  to  see  water  raised 
from  the  bottom  of  a  ravine  to  a  house  high  above  the 
stream.  But  he  can  soon  be  convinced  that  the  principle 
of  the  conservation  of  energy  is  not  violated  by  this  de- 
vice; much  more  water  is  falling  than  rising  all  the  time. 
The  analogy  is  a  close  one;  in  the  animal  there  is  much 


PLANTS    AND    ANIMALS  33 

more  oxidation  than  reduction,  much  more  cleavage  than 
synthesis,  but  some  of  the  energy  made  available  by  the 
decomposition  is  turned  to  account  to  accomplish  a  cer- 
tain amount  of  reconstruction. 

Food. — It  is  now  apparent  that  what  we  mean  by  a  food 
is  generally  something  which  can  be  oxidized  to  yield 
energy  for  the  support  of  the  activities  of  the  living  state. 
In  other  words,  most  food  is  fuel.  But  this  is  not  a  suf- 
ficiently inclusive  definition  for  the  food  of  animals  or 
plants.  We  wish  to  reckon  water  as  a  food  and  we  cannot 
regard  it  as  a  fuel.  There  is  the  same  difficulty  with 
mineral  salts.  We  ought  to  regard  as  food  any  supply 
that  ministers  to  growth  or  repair  as  well  as  to  the  evolu- 
tion of  energy.  Water  is  then  a.  food  because  it  makes 
good  an  unavoidable  loss  which  the  body  suffers. 

The  proteins  occupy  an  interesting  position  among 
foods  because  they  are  necessary  for  the  construction  of 
new  living  matter  and  at  the  same  time  they  are  available 
as  a  source  of  energy.  The  owner  of  a  house  might  have 
a  quantity  of  lumber  brought  to  his  premises  with  a 
view  to  having  an  addition  built.  If  he  then  abandoned 
his  plan  he  might  saw  up  the  boards  and  beams  and  use 
them  for  fuel.  So  we  supply  ourselves  with  proteins 
which  are  especially  adapted  to  constructive  uses  and 
yet  we  consign  most  of  them  to  our  vital  fires.  The 
comparison  is  not  wholly  just;  it  conveys  an  impression 
of  wastefulness  and  improvidence  which  we  should 
scarcely  hold  to  be  valid  in  our  own  case  or  in  that  of 
animals,  in  general. 

Ought  we  to  class  oxygen  among  foods?  This  has 
sometimes  been  done,  but  a  good  reason  can  be  given  for 
not  admitting  it  to  such  rank.  A  food  should  either  be 
a  fuel  or  a  means  of  repair.  Now  oxygen  is  an  agent  of 
destruction,  and  even  though  the  decomposition  is  a 
highly  characteristic  part  of  life  it  is  well  to  keep  oxygen 
in  a  class  by  itself.  We  do  not  regard  the  draft  which 
keeps  the  fire  going  in  a  stove  as  resembling  the  coal 
which  we  must  also  furnish.  To  think  of  the  oxygen 

3 


34  HUMAN    PHYSIOLOGY 

rather  than  the  fuel  us  the  repository  of  energy  waiting 
to  be  set  free  would  be  an  unusual  and  confusing  mental 
practice.  We  shall  therefore  place  oxygen  in  contrast 
with  food  instead  of  extending  the  term  food  to  include 
this  other  requisite  of  the  living  state. 

Summary. — Animals  and  plants  alike,  though  in  un- 
equal degree,  are  engaged  while  they  live  in  turning  the 
latent  or  potential  energy  of  organic  compounds  (fuels) 
into  heat.  Mechanical  work  is  done  by  them  also  and 
is  a  manifestation  of  the  same  great  process.  This 
oxidative  release  of  energy  which  is  coextensive  with  life 
we  call  respiration.  It  would  soon  terminate  because 
of  the  exhaustion  of  the  fuel  supply  of  the  world  if  a 
compensating  process  were  not  going  on.  This  is  photo- 
synthesis, the  manufacture  of  fresh  stores  of  organic 
matter  by  the  higher  plants.  It  cannot  occur  unless 
there  is  a  supply  of  energy  to  drive  it,  and  the  ultimate 
origin  of  this  energy  is  in  the  sun. 

It  is  to  be  borne  in  mind  that  it  is  not  only  animals  for 
whose  ravages  the  higher  plants  have  to  compensate. 
The  lower  plants  are  destructive  in  their  general  tend- 
ency. The  fires  which  men  build  and  those  which  run 
riot  in  the  forests  or  over  the  prairies  destroy  immense 
quantities  of  fuel  which  can  be  recovered  only  by  the 
pigmented  plants  and  in  the  light.  It  seems  an  unequal 
struggle  on  the  part  of  the  leaves.  Yet  for  ages  the 
balance  has  been  fairly  maintained.  It  is  a  question 
whether  human  extravagance  has  now  begun  to  disturb 
it.  Many  think  that  this  is  the  case  and  pleas  for  the 
conservation  of  coal  and  the  extension  of  woodland  areas 
have  become  familiar.  Whenever  the  power  of  falling 
water  is  substituted  for  that  of  steam  we  are  sparing 
organic  fuel,  though  we  are  still  employing  the  energy  of 
the  sun. 

The  Work  of  Bacteria. — The  statement  has  been  made 
that  colorless  plants  act  more  or  less  like  animals  as 
judged  by  the  chief  chemical  effects  which  they  produce. 
It  has  also  been  said  that  they  destroy  the  dead  remains 


PLANTS    AND    ANIMALS  35 

of  the  higher  plants  and  turn  them  into  forms  of  matter 
which  can  reenter  the  life  cycle  of  the  vegetable  world. 
Similarly,  the  colorless  plants  are  the  agents  of  decay 
where  animal  remains  are  concerned.  Especial  atten- 
tion should  be  given  to  the  action  of  some  of  these 
colorless  plants — the  bacteria — upon  the  waste-products 
of  animals. 

Carbon  dioxid  and  water,  the  major  wastes  of  animal 
life,  are  set  free  in  fit  condition  to  be  used  at  once  in 
photosynthesis.  This  is  not  true  of  the  nitrogenous 
excretions  of  animals.  Take,  for  example,  the  compound 
urea  which  is  the  principal  form  in  which  nitrogen  is 
eliminated  by  mammals.  This  is  not  directly  valuable 
to  the  higher  plants  when  it  is  dissolved  in  the  soil  about 
their  roots.  But  there  are  bacteria  which  can  transform 
urea  into  other  substances  beneficial  to  them.  The 
result  is  not  brought  about  by  a  single  reaction  but  by 
stages,  probably  by  three  successive  processes  and 
three  different  kinds  of  organisms. 

Urea  is  first  changed  to  compounds  of  ammonia. 
These  in  their  turn  are  oxidized  to  nitrites.  The  final 
step  is  the  further  oxidation  of  the  nitrites  to  nitrates. 
The  utility  of  the  nitrates  in  protein  synthesis  as  carried 
on  by  green  plants  has  already  been  noted.  A  striking 
demonstration  of  it  occurs  in  connection  with  well 
waters.  These  have  been  long  in  the  earth  and  are 
rich  in  nitrates  as  compared  with  rain  or  surface  waters. 
If  well  waters  are  pumped  into  open  reservoirs  and 
left  standing  in  the  sunlight  they  encourage  the  growth 
of  green  plants  (algae)  to  an  extent  which  is  most  un- 
desirable in  a  public  supply.  If  reservoirs  which  are 
roofed  over  are  provided  for  the  storage  of  such  waters 
no  troublesome  growth  will  develop. 

The  term  Cycle  of  Nitrogen  has  been  used  to  denote 
the  endless  process  in  course  of  which  protein  is  built 
up  by  plants  from  simple  materials  and  with  the  aid  of 
the  solar  energy,  to  be  destroyed  by  the  respiratory 
activity  of  the  plant  itself,  or  of  an  animal,  and  then 


36  HUMAN    PHYSIOLOGY 

to  be  further  degraded,  so  far  as  its  nitrogenous  group- 
ings are  concerned,  by  the  bacteria  of  the  soil.  No 
mention  has  been  made  in  this  discussion  of  the  abound- 
ing atmospheric  nitrogen.  The  free  element  does  not 
arise  in  any  great  quantity  from  organic  decompositions 
nor  does  it  generally  enter  into  the  economy  of  living 
matter  directly  from  the  air.  In  a  few  cases,  which  are 
highly  important,  nitrogen  is  captured  by  bacteria  and 
" fixed"  by  them  as  a  part  of  their  own  substance. 


CHAPTER  III 
CELLS  AND  THEIR  ASSOCIATION 

We  can  imagine  a  giant  of  enormous  stature  and  a 
curious  turn  of  mind  who  might  walk  about  the  earth 
looking  down  upon  the  buildings  which  men  have  made 
as  we  look  at  the  pebbles  in  our  path.  For  such  a  giant 
a  brick  house  might  appear  as  an  object  with  uniformly 
red  surfaces;  the  white  lines  of  the  mortar  might  easily 
be  too  fine  for  his  vision.  He  could  not  observe  the  fact 
that  the  walls  of  the  house  are  made  of  individual  bricks. 
This  might  be  revealed  to  the  colossus  if  he  were  provided 
with  a  microscope  of  suitable  proportions. 

Man  has  had  an  experience  not  unlike  that  which  has 
just  been  suggested.  Through  centuries  of  eager  in- 
tellectual life  he  could  not  see  that  the  green  surface  of  a 
leaf  is  a  mosaic  of  minute  units.  He  could  not  re- 
solve the  substance  of  an  animal  body  into  its  con- 
stituent parts.  The  modern  microscope  had  to  be 
perfected  before  this  knowledge  could  be  compassed. 
More  than  two  hundred  and  fifty  years  ago  there  were 
drawings  and  descriptions  of  the  appearance  of  parts 
of  plants  and  animals  under  high  magnification.  Some 
of  these  were  surprisingly  accurate.  But  the  lenses 
available  in  the  seventeenth  century  were  poor  affairs 
which  blurred  and  distorted  the  images  which  they 
formed.  The  investigator  had  to  make  the  best  of 
these  faulty  aids  and,  in  addition,  to  ignore  so  far  as  he 
could  the  fringes  of  rainbow  colors  which  bordered  every 
object  of  his  study. 

Great  improvements  in  lenses  were  made  about  the 
year  1830,  and  biologists  hastened  to  employ  the  new 
instruments.  A  fruitful  period  of  research  and  publica- 

37 


38  HUMAN    PHYSIOLOGY 

tion  followed.  Within  a  decade  the  scientific  world  had 
fairly  grasped  the  conception  which  we  know  as  the  Cell 
Theory.  What  this  is  and  what  it  implies  must  now  be 
shown  with  some  care.  What  do  we  mean  when  we 
speak  of  a  cell? 

It  must  be  admitted  at  once  that  the  word  cell  is  not 
particularly  appropriate  in  the  sense  in  which  it  has 
come  to  be  used.  A  cell  is  properly  a  walled  space,  as 
in  the  prison  or  the  honeycomb.  In  biologic  parlance 
a  cell  is  a  small  parcel  of  living  matter.  It  is  easy  to 
explain  how  the  name  began  to  be  used.  Dead  and  dry 
vegetable  substance,  like  cork  which  was  one  of  the  first 
forms  studied,  has  a  structure  which  can  be  correctly 
described  as  cellular.  There  are  myriads  of  micro- 
scopic spaces  set  apart  by  walls.  The  arrangement  was 
pictured  carefully  by  Robert  Hooke  about  the  year 
1665.  The  cells  seen  by  this  early  worker  were  actual 
cavities,  but  we  have  come  to  employ  the  term  for  the 
separate  packets  of  organized  material  which  occupied 
these  cavities  when  the  tissue  was  living  and  growing. 

The  microscope  shows  that  bits  of  animal  matter  also 
consist  of  assembled  units  of  about  the  same  average 
size  as  those  of  plants.  But  the  so-called  cells  in  the 
animal  are  not  commonly  isolated  by  rigid  and  durable 
partitions.  Between  neighboring  ones  the  intervals  may 
be  occupied  by  fluid,  or  partially  taken  up  by  fibers,  or 
there  may  be  more  or  less  compact  deposits,  but  in 
general  one  receives  the  impression  from  observation  of 
animal  cells  that  they  are  often  in  practical  contact 
one  with  another  and  that  it  is  curious  that  they  do  not 
run  together.  There  is  reason  to  suppose  that  the 
surface  of  each  cell  presents  a  film  somewhat  different  in 
character  from  the  interior;  this  is  sometimes  spoken 
of  as  the  cell-membrane,  but  it  is  of  extreme  delicacy  and 
not  to  be  compared  with  the  substantial  walls  which 
bound  the  cells  of  plants. 

Cells  vary  widely  in  size.  There  are  probably  more 
which  measure  less  than  Kooo  in<:h  in  diameter  than  there 


CELLS    AND    THEIK    ASSOCIATION  39 

are  which  exceed  this  scale.  The  name  protoplasm  is 
given  to  the  characteristic  material  of  which  cells  are 
composed.  In  most  cases  there  can  be  demonstrated 
in  a  cell  an  internal  mass  which  seems  denser  than  the 
rest.  This  is  called  the  nucleus.  We  are  in  the  habit 
of  speaking  of  the  cell  as  a  small  mass  of  living  matter 
but  we  are  compelled  to  admit  when  questioned  that  we 
do  not  know  what  proportion  of  the  whole  is  truly  alive. 
This  is  a  stimulating  but  rather  hopeless  line  of  inquiry. 
When  cells  from  different  parts  of  the  body  are  com- 
pared we  can  recognize  that  some  show  special  adapta- 


Fio.  1. — To  contrast  the  empty  "cells"  of  a  dry,  woody  tissue,  en- 
closed by  substantial  walls,  with  the  "cells"  of  a  soft,  animal  tissue 
which  are  separate  parcels  of  living  matter.  They  are  related  to  the 
other  somewhat  as  casts  to  their  moulds. 

tion  to  particular  uses  while  others  are  obviously  of  a 
more  primitive  type.  Qells  that  can  be  regarded  as 
primitive  suggest  to  one  that  the  standard  form  is  the 
sphere.  Yet  this  form  is  seldom  perfectly  realized, 
chiefly  because  cells  which  are  pressed  together  naturally 
come  to  have  flattened  surfaces  at  places  of  contact. 
Thus  they  are  like  grapes  that  have  been  packed  too 
tightly.  We  ought  to  think  of  cells  as  of  a  very  soft 
consistency.  The  structural  strength  of  the  body  as  a 
whole  is  gained  through  the  development  in  it  of  deposits 
and  fibers  which  are  not  cellular  but  intercellular  in 
nature. 

Tissues. — Before  cells  were  recognized  as  characteristic 
units,  into  which  living  matter  can  be  resolved,  anato- 


40 


HUMAN    PHYSIOLOGY 


mists  knew  that  the  bodies  of  animals  were  composed 
of  various  types  of  material,  the  tissues.  Thus  we 
say  that  bone,  muscle,  skin,  the  substance  of  the  nervous 
system,  and  blood  are  tissues.  In  the  light  of  the  cell 
theory  we  are  inclined  to  think  of  a  tissue  as  a  collec- 
tion of  similar  cells.  This  is  not  wholly  satisfactory, 
for  the  character  of  a  given  tissue  is  often  more  dependent 
on  the  nature  and  amount  of  the  intercellular  matter 
mentioned  above  than  on  the  peculiarities  of  the  cells. 
Therefore,  it  is  best  to  regard  a  tissue  as  consisting  of 
associated  cells  and  intercellular  accumulations.  In 
some  cases,  as  in  the  skin,  the  intercellular  deposit  is 
slight;  at  the  other  extreme  we  have  cartilage  and  bone 


FIG.  2. — To  emphasize  the  difference  between  an  epithelial  tissue 
(at  the  left)  in  which  the  cells  are  closely  packed  and  a  form  of  connec- 
tive tissue  (cartilage)  consisting  mainly  of  an  intercellular  deposit. 

where  the  cells  form  but  a  very  small  fraction  of  the 
total  mass. 

It  was  suggested  a  moment  ago  that  blood  is  a  tissue. 
This  may  not  be  a  familiar  idea  nor  one  that  is  universally 
acceptable.  But  it  will  be  noted  that  blood  conforms 
with  the  terms  of  our  definition.  It  contains  cells  of 
standard  kinds  with  intervening  material  which  in  this 
instance  is  a  liquid.  There  is  more  or  less  intercellular 
fluid  in  almost  any  variety  of  tissue. 

Four  conspicuous  orders  of  tissues  may  be  set  apart. 
They  are  (1)  the  epithelial,  (2)  the  connective,  (3)  the 
contractile,  and  (4)  the  nervous.  Those  of  the  first  class 
(epithelial  tissues)  are  the  surface  tissues,  the  coverings 


CELLS    AND    THEIR    ASSOCIATION  41 

and  linings  of  the  organs.  An  epithelium  may  consist 
of  a  single  layer  of  cells  or  of  more  than  one.  The  thin- 
nest developments  of  this  character  are  exemplified  in 
the  partitions  between  the  air  and  the  blood  in  the 
lungs.  In  calloused  portions  of  the  skin  the  number  of 
layers  of  cells  is  large  and  those  on  the  outer  surface  are 
lifeless,  flattened,  and  dry.  Epithelial  cells,  as  a  rule, 
keep  growing  and  subdividing  throughout  life  to  make 
good  the  loss  by  degeneration  and  detachment  that  is 
constantly  going  on. 

Connective  tissues  have  the  general  function  which  the 
name  indicates.  Bone  is  an  example  and  what  the 
skeleton  does  for  the  body  as  a  whole  is  done  for  each 
organ  by  a  supporting  web  of  material  belonging  to  this 
class.  Under  this  head  we  may  mention  the  tendons 
which  join  muscles  with  bones  and  the  ligaments  which 
unite  the  bones  at  the  movable  joints.  As  already 
suggested  the  cells  of  connective  tissue  are  not  much  in 
evidence.  It  is  the  intercellular  substances,  in  the  form 
of  fibers,  concrete,  or  mineralized  deposits  that  fit  such 
tissues  to  serve  their  purpose. 

Contractile  tissues  are  those  which  produce  movement 
by  an  energetic  change  of  form.  They  are  able  to  do 
this  because  they  can  derive  power  from  certain  chemical 
compounds  which  they  decompose.  The  problems 
of  muscle  physiology  confront  us  here.  It  is  well  to 
point  out  that  the  tissues  which  display  the  property 
of  contractility  are  heat-producing  tissues  at  the  same 
time. 

The  nervous  tissues  are  in  many  respects  the  most 
remarkable  and  baffling  of  all.  An  attempt  to  deal 
with  them  will  be  made  later  on;  it  may  be  said  at  this 
point  that  their  chief  service  is  to  maintain  the  relation- 
ship between  the  sense-organs  and  the  contractile  tissues. 
In  other  words  their  function  is  that  of  coordination. 
This  seems  a  very  inadequate  statement  when  we  con- 
sider the  mysterious  correlation  between  the  brain  and 
our  consciousness,  yet  we  shall  find  that  most  of  the 


42  HUMAN    PHYSIOLOGY 

strict  physiology  of  the  nervous  system  can  be  harmon- 
ized with  it. 

The  contractile  and  the  nervous  tissues  have  been 
called  the  " master  tissues"  of  the  body.  It  is  quite 
clear  that  they  are  more  essentially  living  and  active 
than  the  others.  One  can  imagine  non-living  substi- 
tutes for  bones  and  tendons — and,  in  fact,  such  are 
sometimes  used — but  one  cannot  imagine  any  lifeless 
contrivance  that  could  replace  a  muscle  or  a  section 
of  the  nervous  system.  As  to  the  epithelial  tissues, 
it  might  be  supposed  that  these  would  be  found  purely 
passive  but  this  turns  out  not  to  be  the  case.  It  is 
through  the  epithelial  expanses  that  transfers  of  food 
and  waste  go  on  between  organisms  and  their  environ- 
ment. The  processes  involved,  secretion  and  absorp-. 
tion,  cannot  be  reproduced  with  non-living  membranes. 

Free-living  Cells. — A  cell  from  the  body  of  one  of  the 
higher  animals  cannot  continue  to  live  when  separated 
from  its  fellows.  But  nature  abounds  in  organisms 
which  are  single  cells.  These  may  be  reckoned  accord- 
ing to  their  nutritional  requirements  in  different  instances 
as  either  plants  or  animals.  So  the  bacteria  are  con- 
sidered to  be  plants  because  they  can  live  on  classes  of 
supplies  which  are  simpler  in  their  constitution  than 
those  which  animal  life  ordinarily  demands.  The 
Infusoria  of  stagnant  water,  which  are  seen  to  devour 
organic  matter  and  whose  lively  habits  constantly  re- 
mind us  of  the  higher  forms,  are  regarded  as  animals. 
But,  as  has  been  stat'ed  before,  the  distinctions  between 
plants  and  animals  have  only  a  limited  value  when 
applied  to  the  lower  orders  of  life. 

A  free-living  cell  of  the  animal  type  has  probably  all 
the  activities  which  have  been  described  in  the  preceding 
chapter  as  characteristic  of  animals  in  general.     That. 
is  to  say,  it  appropriates  supplies  of  complex  matter,  - 
using  some  to  promote  its  own  growth  but  more  to  furnish, 
the  energy  for  movement  and  heat  production.     This 
energy  it  makes  available  by  a  respiratory  process  in 


CELLS    AND    THEIR    ASSOCIATION  43 

which  oxygen  is  consumed  and  simple  end-products 
formed.  Movement  in  the  independent  cell,  as  in  the 
larger  organisms,  is  the  expression  of  contraction.  Some 
explanation  of  this  term  is  desirable. 

Contraction,  as  the  biologist  understands  it,  does  not 
mean  diminution  of  volume.  It  is  thus  different  from 
contraction  in  the  physical  sense.  When  the  mercury 
in  the  thermometer  tube  contracts  under  the  influence 
of  cold  there  is  an  actual  reduction  of  the  space  which  the 
metal  occupies.  When  a  muscle  contracts  it  can  be 


FIG.  3. — The  purpose  of  this  diagram  is  to  assist  the  student  in 
gaining  notions  of  scale.  The  large  circle  stands  for  the  cross-section 
of  a  fine  hair.  Its  diameter  is  supposed  to  be  3^00  inch.  Within  it 
(bl)  is  a  red  blood  corpuscle,  ^200  in('h  across.  The  budding  yeast-cell 
(y)  is  of  a  similar  order  of  magnitude.  Bacteria  (bact.)  are  a  good  deal 
smaller. 

shown  that  the  volume  is  unchanged;  there  has  been  a 
shortening  in  one  dimension  but  a  compensating  thick- 
ening in  others.  Changes  of  form  which  are  observed  in 
single  cells  are  doubtless  of  this  kind,  save  in  those 
cases  in  which  water  enters  or  leaves.  This  exceptional 
possibility  we  shall  not  be  obliged  to  consider  in  the 
present  treatment  of  the  subject. 

Many  of  the  facts  of  life  which  we  assume  for  free- 
living  cells  can  hardly  be  demonstrated  but  are  rather 
inferred  from  what  we  know  of  higher  organisms.  The 


44  HUMAN    PHYSIOLOGY 

production  of  heat,  for  example,  is  something  we  can 
scarcely  hope  to  measure  when  only  one  cell  is  concerned, 
but  we  cannot  doubt  that  it  is  going  on  because  we  know 
that  it  is  evident  when  sufficiently  large  numbers  of 
active  cells  are  massed.  The  phenomena  we  can  most 
easily  make  out  in  a  study  of  solitary  cells  are  the  taking 
of  food,  motion,  and  reproduction.  A  word  may  be 
said  about  the  last-named  manifestation  of  life. 

Cell  Reproduction. — A  cell  which  is  well  nourished  and 
otherwise  in  a  favorable  condition  soon  attains  to  a  size 
which  marks  the  limit  for  its  growth.  Instead  of  in- 
creasing further  in  bulk  it  cleaves  into  two  parts  which 
are  complete  and  living  cells.  The  cells  of  the  new 
generation  are  at  first  undersized  but  these  in  their  turn 
grow  to  the  standard  of  the  species.  It  is  curious  to 
reflect  that  there  is  a  kind  of  physical  immortality  be- 
longing to  the  one-celled  organisms.  Cell-division  is 
obviously  not  death.  Many  members  of  the  family  are 
killed  by  accidental  means,  but  they  do  not  seem  fated 
to  grow  old  and  perish  from  any  intrinsic  property  of 
their  own.  The  cell  which  is  living  to-day  has  descended 
from  a  line  of  ancestral  cells  no  one  of  which  has  ever 
died. 

This  assertion  may  be  made  with  reference  to  the 
cells  of  higher  plants  and  animals  as  well  as  to  those  which 
live  alone.  No  cell  that  is  living  to-day  has  ever  lost  a 
direct  ancestor  by  death.  Let  us  see  in  what  sense  this 
is  true.  Take,  for  example,  a  cell  in  the  human  skin. 
Its  neighbors  at  the  surface  are  rapidly  dying,  shrivel- 
ling, and  being  cast  off.  This  will  be  its  own  probable 
fate.  But,  if  we  look  backward  instead  of  forward, 
we  recognize  that  this  selected  cell  was  formed  a  short 
time  ago  by  the  cleavage  of  a  preexisting  cell.  The 
companion  formed  at  the  same  time  may  have  perished 
but  the  fact  remains  that  the  ancestor  did  not  die.  The 
same  may  be  said  of  the  parent  cell  in  the  previous 
generation. 

As  we  trace  the  succession  backward  our  attention 


CELLS   AND    THEIR   ASSOCIATION  45 

becomes  fixed  at  last  upon  some  cell  in  the  embryo  and 
eventually  upon  the  single  cell,  the  fertilized  egg  or 
ovum,  from  which  all  the  countless  host  of  cells  in 
the  mature  body  are  descended.  This  lived  before  the 
individual  whom  we  have  chosen  to  picture.  "Omnis 
cellula  e  cellula,"  said  Virchow  in  the  last  century. 
"Omne  vivum  ex  ovo,"  Harvey  had  written  two  hundred 
years  before.  Either  of  these  famous  sayings  will  serve 
to  bring  home  to  us  the  fascinating  thought  of  the 
uninterrupted  living  bonds  which  unite  all  forms  of 
the  present  with  the  most  distant  past.  In  retrospect 
these  threads  of  life  seem  endless,  yet  any  one  of  them 
may  terminate  at  any  moment. 

When  we  think  of  our  ancestors  who  have  died — 
perhaps  fourteen  of  them  in  a  hundred  years — we  must 
be  impressed  by  the  disproportion  between  the  mass  of 
their  tissues  which  perished  and  the  infinitesimal  survival 
in  ourselves  of  what  they  transmitted  to  us.  Someone 
has  said  that  the  body  is  like  a  great  lantern,  serving 
primarily  to  save  the  tiny,  trembling  flame  of  the  germi- 
nal life  from  being  extinguished.  It  is  a  lantern  which 
wears  out  and  only  the  flame  can  be  saved  to  burn  for  a 
time  in  a  fresh  protective  shell.  One  contrast  between 
single-celled  and  many-celled  forms  of  life  will  now  be 
clear:  in  the  former  the  whole  substance  of  one  genera- 
tion may  live  to  constitute  the  next,  in  the  latter  the 
accumulated  cells  of  the  body  die  and  under  the  most 
favorable  conditions  leave  but  the  minutest  part  to 
represent  the  stock. 

Another  difference  between  the  unicellular  and  the 
more  highly  developed  organisms  is  associated  with  sex. 
A  single  bacterium  may  give  rise  to  two,  and  these  to 
four  descendants,  and  so  on.  A  solitary  member  of 
any  of  the  more  advanced  types  must  mate  with  another 
of  opposite  sex  if  the  species  is  to  be  reproduced.  The 
fertilized  ovum,  before  mentioned,  which  develops  into 
the  embryo  and  so  into  the  mature  individual  may  be 
called  one  cell,  but  it  is  more  truly  a  composite  of  two 


46  HUMAN    PHYSIOLOGY 

half-cells,  one  furnished  by  the  male  and  one  by  the 
female.  Thus  the  animal  of  the  higher  sort  has  two 
parents,  four  grandparents,  eight  ancestors  in  the  next 
generation,  the  numbers  soon  becoming  enormous  as  we 
reckon  backward.  The 'bacterium,  on  the  other  hand, 
has  but  one  ancestor  in  a  generation. 

The  case  of  the  one-celled  animal  forms  is  not  so  simple 
as  may  have  appeared  from  the  unqualified  statements 
that  have  been  made.  Close  observation  of  these 
orders  of  life  has  shown  that,  while  the  multiplication 
of  cells  by  cleavage  is  the  common  process  among  them, 
in  many  instances  the  fusion  of  two  cells  into  one  (con- 
jugation) is  a  possibility.  When  it  takes  place  there  is 
reason  to  believe  that  a  more  vigorous  and  enduring 
organism  is  produced.  It  is  a  means  of  rejuvenation. 
Conjugation  of  free-living  cells  seems  a  prophecy  of 
sex  as  realized  in  the  higher  varieties  of  plants  and 
animals.  In  these  we  know  that  the  union  of  the 
two  germ-cells  gives  rise  to  an  organism  which  we  call 
young,  meaning  that  it  has  powers  of  growth  and 
development  which  the  parents  have  largely  lost. 

We  ought  now  to  compare  somewhat  fully  the  situa- 
tion of  a  cell  that  exists  alone  and  one  which  acts  as  a 
member  of  the  great  community  making  up  the  body  of 
one  of  the  larger  animals.  In  the  first  place,  a  direct 
consequence  of  the  size  of  such  a  body  is  that  the  great 
majority  of  the  cells  are  submerged  and  surrounded  by 
their  fellows  instead  of  maintaining  immediate  relations 
with  the  outside  world.  Special  provision  must  be 
made  for  bringing  food  to  such  cells  and  relieving  them 
of  waste.  These  purposes  are  served  in  the  higher 
animals  by  liquid  media,  the  blood  and  the  lymph. 

The  blood  is  confined  to  a  system  of  vessels  in  which  it 
moves  steadily  in  one  direction.  Only  those  cells  which 
line  the  vessels  are  actually  bathed  by  the  blood.  All 
other  cells — and  this  means  the  vast  majority — are 
removed  from  direct  contact  with  the  blood  but  have 
around  and  between  them  the  second  fluid,  the  lymph. 


CELLS    AND    THEIR    ASSOCIATION  47 

From  this  they  draw  nutriment  and  oxygen;  to  it  they 
discharge  carbon  clioxid  and  other  products  of  their 
activity.  The  lymph  adjacent  to  any  one  cell  is  a  very 
limited  quantity  and  if  there  were  no  provision  for  its 
renewal  its  usefulness  would  soon  be  at  an  end.  But 
the  blood  is  flowing  close  by  in  capillaries  whose  thin 
walls  scarcely  impede  the  passage  of  dissolved  gases  and 
other  materials  between  the  blood  and  the  lymph. 
By  a  continuous  exchange  between  the  two  fluids  the 
lymph  is  relieved  of  cell  waste  and  held  to  a  standard 
composition  as  regards  oxygen  and  food. 


FIG.  4.— To  suggest  the  confinement  of  the  blood  within  definite 
vessels  (bb).  The  lymph  occupies  the  dotted  region  between  these 
blood-vessels  and  the  tissue-cells. 

One  of  the  striking  facts  we  note  when  we  compare 
free-living  and  associated  cells  is  expressed  by  the  term 
" division  of  labor"  which  is  used  with  reference  to  the 
latter.  The  cells  are  evidently  of  different  orders  and 
specially  adapted  to  particular  functions.  Those  of 
muscle  are  eminently  contractile.  Those  of  the  nervous 
tissues  have  most  highly  developed  the  property  of 
conduction.  Some,  as  in  the  skin,  have  it  their  chief 
duty  to  provide  hosts  of  descendants  whose  dead  remains 
may  form  a  protective  covering.  The  specific  service  of 
the  cells  in  the  connective  tissues  is  to  elaborate  inter- 
cellular deposits. 


48  HUMAN    PHYSIOLOGY 

Along  with  specialization  such  as  has  been  illustrated 
the  cells  lose  some  of  the  primitive  endowments.  A  cell 
of  muscle  or  of  the  nervous  system  cannot  feed  upon  all 
sorts  of  food  particles  like  the  roving  infusorium.  It  is 
limited  to  the  use  of  dissolved  foods  which  must  be  of  a 
few  standard  types.  The  power  of  movement  is  not 
preserved  in  the  majority  of  cells  in  the  higher  animals. 
It  is  the  peculiar  property  of  the  muscular  tissues. 
When  the  body  is  in  motion  it  is  these  elements  which 
are  at  work  and  all  the  rest  are  moved  by  them.  The 
result  of  physiologic  division  of  labor,  combined  with 
the  removal  of  most  cells  from  direct  relations  with  the 
outside  world,  is  the  absolute  dependence  of  each  cell 
upon  the  contributions  of  others  for  its  continued 
existence. 

One  of  the  capacities  which  may  be  lost  in  connection 
with  specialization  of  structure  and  function  on  the  part 
of  cells  is  that  of  reproduction.  We  have  seen  that  this 
is  retained  by  epithelial  units;  its  persistence  in  the  hair 
and  the  nails  is  most  remarkable.  But  it  is  not  retained 
by  the  most  conspicuous  kind  of  muscle  in  the  body. 
The  enlargement  of  a  muscle  in  consequence  of  exercise 
is  due  to  increased  size  of  the  individual  units  and  not 
to  their  multiplication.  So,  too,  in  the  nervous  system: 
the  wonderful  advances  in  the  working  of  the  brain  from 
infancy  to  maturity  are  not  assisted  by  the  addition  of  a 
single  cell  to  the  number  originally  present  but  only  by 
the  organization  of  this  collection. 

Elements  of  Anatomy 

Our  natural  interest  will  center  in  the  physiology  of 
man,  and  before  we  go  farther  the  outlines  of  human 
anatomy  may  be  very  briefly  suggested.  The  skin, 
which  forms  the  surface  of  the  body,  is  an  epithelium 
of  many  layers.  Beneath  it  there  is  loose  connective 
tissue,  more  or  less  rich  in  fat.  Deeper  still  we  find  the 
muscles.  The  bones,  articulated  as  the  skeleton,  give 


CELLS    AND    THEIR   ASSOCIATION  49 

support  and  fixed  proportions  to  the  whole.  In  various 
places  between  the  skin  and  the  bones  we  find  the  white 
nerves  and  the  large  blood-vessels.  These  are  of  two 
classes,  arteries  which  carry  blood  away  from  the  heart 
and  veins  whicn  conduct  blood  back  toward  that  organ. 
The  arteries  generally  lie  at  some  depth  below  the  surface 
while  veins  are  situated  at  all  depths,  the  superficial  ones 
being  visible  through  the  skin. 

The  Body  Cavities. — The  features  which  have  been 
mentioned  are  all  that  need  be  included  in  a  simple 
description  of  what  is  to  be  seen  in  dissecting  an  arm 
or  a  leg,  but  the  case  is  different  with  the  head  or  the 
trunk.  In  these  are  what  we  speak  of  as  cavities;  the 
use  of  the  word  needs  to  be  carefully  defined.  The 
student  is  apt  to  imagine  that  the  so-called  body  cavities 
contain  more  or  less  vacant  space.  This  is  not  the  actual 
condition.  They  are  only  potential  cavities  which  be- 
come real  ones  when  their  contents  have  been  removed. 
In  life  they  are  completely  filled  by  the  organs,  plus  a 
small  quantity  of  fluid. 

In  the  head  the  principal  cavity  is  that  which  we  call 
the  cranial  one,  the  space  which  accommodates  the  brain. 
It  is  bounded  by  bones  pierced  here  and  there  by  small 
openings  for  the  nerves  and  at  one  place  by  the  larger 
orifice  through  which  the  spinal  cord  descends.  This 
last-mentioned  opening  is  at  the  base  of  the  skull  and 
leads  to  a  tunnel  made  by  the  successive  bony  arches 
of  the  vertebrae.  In  the  trunk  we  distinguish  two  main 
cavities,  that  of  the  thorax  within  the  sweep  of  the  ribs 
and  that  of  the  abdomen  below.  The  partition  between 
them  is  the  diaphragm,  a  sheet  composed  partly  of 
muscular  and  partly  of  connective  tissue  which  has  the 
form  of  a  tolerably  high  dome  and  therefore  subtracts 
much  space  from  the  apparent  size  of  the  thorax  and  adds 
it  to  the  abdomen.  Below  the  abdomen  and  within  the 
circle  of  the  hip-girdle  is  the  small  pelvic  cavity. 

A  striking  fact  about  the  body  cavities  is  that  the 
organs  which  they  contain  are  not  attached  to  the  en- 


50 


HUMAN    PHYSIOLOGY 


compassing  walls  save  at  a  few  places.  For  the  most 
part  the  surfaces  of  the  organs  bear  upon  surfaces  of 
the  body  walls  but  do  not  adhere  to  them.  The  arrange- 
ment permits  a  certain  amount  of  gliding  of  one  upon 
the  other.  There  cannot  normally  be  a  separation  be- 


FIG.  5. — Suggesting  the  thoracic  and  abdominal  cavities  parted  by 
the  diaphragm.  The  abdominal  viscera  are  drawn  upward,  creating 
a  space  above  the  bladder  where  none  normally  exists. 


tween  them  -since  this  would  involve  the  creation  of  a 
vacuum.  The  relation  between  an  organ  and  the 
opposing  body  wall  is  much  like  that  between  two 
plates  of  glass  which  have  been  moistened  and  laid 
together.  One  will  slide  freely  upon  the  other,  but  it 


CELLS    AND    THEIR    ASSOCIATION  51 

requires  great  force  to  pull  them  apart  until  air  begins 
to  penetrate  between  them. 

The  thorax  is  divided  vertically  into  right  and  left 
sections  by  a  development  of  connective  tissue  called 
the  mediastinum.  In  this  is  suspended  the  heart  sur- 
rounded by  a  sac,  the  pericardium.  On  either  side  of 
the  mediastinum  is  a  typic  cavity,  lined  with  a  smooth, 
moist  membrane  which  is  in  contact  with  another 
membrane  of  the  same  nature  covering  the  lung.  The 
linings  of  these  two  cavities  and  the  coverings  of  the 
two  lungs  are  spoken  of  as  the  layers  of  the  pleura. 

The  abdominal  cavity  is  lined  by  a  membrane  similar 
to  the  pleura  and  called  the  peritoneum.  The  principal 
organs  contained  in  the  abdomen  are  those  belonging 
to  the  digestive  system.  The  central  feature  of  this 
system  is  the  alimentary  canal  which,  as  a  matter  of  fact, 
is  not  strictly  confined  to  the  abdominal  cavity  since  it 
begins  at  the  mouth,  extends  through  the  thorax,  and 
at  its  lower  termination  traverses  the  pelvis  to  open  at 
the  anus.  Nevertheless,  the  great  part  of  the  digestive 
tract  is  in  the  abdomen.  The  liver  and  the  pancreas 
are  appended  to  the  canal  and  the  spleen  is  associated 
with  it  though  perhaps  less  directly.  The  kidneys  are 
exposed  to  view  when  the  organs  of  digestion  are  re- 
moved from  the  abdominal  cavity;  they  lie  behind  the 
peritoneum  and  are  best  thought  of  as  belonging  to  the 
back  rather  than  to  the  abdomen. 

In  the  pelvis  there  are  found  the  terminal  portion  of 
the  alimentary  canal,  as  already  noted,  the  urinary 
bladder,  and  the  reproductive  organs.  The  peritoneum 
intervenes  between  the  pelvic  cavity  and  that  of  the 
abdomen  above.  Details  will  be  added  as  we  discuss 
the  particular  organs  which  have  merely  been  mentioned 
at  this  place. 

Anatomical  Terms. — It  will  be  well  to  define  here  a 
few  terms  of  anatomy  which  must  often  be  used.  Right 
and  left  have  their  ordinary  meaning.  Dorsal  signifies 
toward  the  back  and  ventral  is  its  opposite  (venter,  the 


52  HUMAN    PHYSIOLOGY 

belly).  There  is  an  unfortunate  confusion  in  regard  to 
the  employment  of  anterior  and  posterior.  They  have 
been  much  used  as  though  equivalent  to  ventral  and 
dorsal  respectively.  But  another  usage  which  appears 
more  desirable  is  to  define  anterior  as  toward  the  head, 
posterior  being  the  reverse.  Anterior  is  then  the  same 
as  superior,  posterior  is  synonymous  with  inferior.  If 
we  adhere  to  the  practices  outlined  we  shall  have  no 
difficulty  in  comparing  the  structural  relations  in  the 
lower  animals  with  those  in  man.  Such  difficulty  is 
experienced  where  the  other  significance  is  given  to 
anterior  and  posterior;  as  an  animal  walks  the  anterior 
parts  precede  the  posterior  but  in  the  erect  position  the 
ventral  precedes  the  dorsal. 


CHAPTER  IV 
CONTRACTILE  TISSUES 

It  is  a  peculiar  fact  about  physiology  that  very  different 
orders  of  presentation  have  commended  themselves  to 
different  writers  and  have  been  used  with  success. 
Whatever  one  chooses  to  place  first,  one  is  likely  soon  to 
wish  that  the  student  were  in  possession  of  some  other 
part  of  the  subject  to  serve  him  as  a  background.  But 
there  is  much  to  be  said  in  favor  of  the  introduction 
early  in  a  book  of  the  physiology  of  movement.  We 
have  to  reckon  with  it  in  all  the  remaining  sections  of  our 
survey.  So  in  the  present  instance  we  shall  take  up  the 
question  of  motion  at  this  time. 

It  has  been  said  previously  that  most  of  the  movements 
executed  by  animal  cells  are  the  expression  of  contraction 
as  that  term  is  understood  by  the  biologist.  Such  move- 
ments may  be  carried  out  by  single  cells  or  by  tissues 
composed  of  cells  whose  action  is  concerted.  We  could 
know  nothing  about  the  behavior  of  single  cells  if  we 
had  not  the  assistance  of  the  microscope.  Thanks  to 
that  instrument  we  have  found  out  that  the  two  ex- 
hibitions of  contractility  which  are  most  common 
among  one-celled  animal  forms  are  to  be  seen  also  in  the 
higher  organisms.  These  are,  respectively,  ameboid  and 
ciliary  movement. 

Ameboid  Movement. — This  manifestion  of  the  con- 
tractile property  takes  its  name  from  a  single-celled 
aquatic  animal,  the  ameba.  It  is  one  of  the  simplest 
types  conceivable,  a  minute  mass  of  jelly-like  substance 
with  a  nucleus  which  confirms  its  right  to  rank  as  a  cell. 
It  is  usually  colorless  and  transparent  except  for  numer- 
ous granules  within.  Its  movement  consists  in  the  most 

53 


54 


HUMAN    PHYSIOLOGY 


irregular  changes  in  outline,  the  border  at  certain  places 
drawing  in  toward  the  center  and  at  others  pushing  out 
in  blunt  protrusions.  When  one  first  watches  an  ameba 
the  impression  is  that  extension  rather  than  contraction 
is  the  characteristic  of  the  movement  displayed.  Closer 
study  shows  that  the  positive  phase  is  really  contraction. 
When  a  process  runs  out  briskly  while  the  remainder 
of  the  cell  appears  to  be  at  rest  the  natural  inference 
is  that  the  process  itself  is  active.  But  the  fact  seems 
rather  to  be  that  the  main  mass  of  the  cell  is  exerting  a 


FIG.  6. — Ameba.     (From  Calkins'  "Biology,"  Courtesy  of  Henry 
Holt  &  Co.  Publishers.) 

pressure  which  drives  or  spurts  out  the  process.  The 
reader  may  have  seen  small  rubber  balls  with  faces 
painted  on  them  and  tongues  of  thin  rubber  which  dart 
out  when  the  balls  are  squeezed.  The  protrusion  in  this 
case  is  probably  the  same  in  principle  as  that  which 
obtains  with  the  ameba;  pressure  in  one  part  results  in 
extension  in  another,  the  moving  part  being  passive  and 
the  real  source  of  the  power  not  being  apparent. 

Uniform  conditions  of  contraction  in  a  cell  like  the 
ameba  tend  to  bring  it  into  a  spheric  form  for  this  is 


CONTRACTILE    TISSUES  55 

the  shape  which  presents  the  minimum  surface.  Strong 
irritation  of  the  ameba  causes  it  to  assume  this  form. 
It  is  in  fact  a  general  law  of  contraction  that  the  elements 
acting  approach  a  spheric  form  as  a  geometric  limit. 
When  the  ameba  begins  to  put  out  processes  and  resume 
movement  after  a  period  of  intense  contraction  it  is  to 
be  supposed  that  the  pressure  directed  inward  at  certain 
points  has  diminished;  thereupon  the  pressure  at  other 
places  is  no  longer  counterbalanced  and  risings  of  the 
surface  occur  in  the  areas  of  lowered  resistance. 

Ameboid  movement  is  exemplified  in  the  bodies  of  the 
higher  animals,  including  our  own,  by  cells  found  in  the 
blood  and  called  leucocytes.  The  word  means  "  white 
cells"  and  has  been  applied  because  these  cells  are 
contrasted  with  the  much  more  numerous  red  corpuscles 
of  the  blood  by  their  lack  of  pigment.  Under  favorable 
conditions  many  of  them  change  their  forms  in  a  manner 
which  is  highly  suggestive  of  the  free-living  amebse  and 
it  was  maintained  at  one  time  that  the  leucocytes  were 
parasites  rather  than  normal  body  cells.  It  is  now 
established  that  they  are  both  normal  and  valuable  in 
the  organism. 

Cells  which  have  the  ameboid  character  have  usually 
the  power  to  enclose  all  manner  of  foreign  particles  with 
which  they  come  into  contact.  It  is  by  such  means  that 
the  aquatic  amebse  secure  food.  A  similar  capacity 
is  observed  in  the  case  of  the  leucocytes;  they  also  sub- 
merge in  their  own  substance  various  small  bodies  which, 
in  favorable  instances,  may  be  digested  and  entirely 
obliterated.  This  action  has  a  peculiar  importance 
because  bacteria  are  frequently  devoured  in  this  way 
and  so  we  count  the  leucocytes  as  defenders  of  the  organ- 
ism against  mischief-making  invaders.  The  process  in 
course  of  which  bacteria  are  engulfed  and  destroyed 
is  called  phagocytosis  which  means  "scavenging." 
Around  a  threatened  spot,  such  as  a  wound  containing 
dirt,  leucocytes  in  vast  numbers  are  massed  in  the 
tissues.  The  student  marvels  that  they  should  be 


56  HUMAN    PHYSIOLOGY 

gathered  so  surely  at  the  very  place  where  their  presence 
is  required,  but  we  do  not  have  to  assume  anything  like 
intelligence  on  the  part  of  these  cells  to  account  for  the 
facts.  We  are  probably  to  suppose  that  conditions  arise 
in  such  regions  which  arrest  the  leucocytes  brought  into 
the  area  by  the  flowing  blood. 

A  large  proportion  of  the  leucocytes  which  are  thus 
concerned  in  contending  with  an  infection  are  found 
not  to  be  inside  but  outside  the  blood-vessels.  How 
they  made  their  escape  is  a  natural  question.  Direct 
observation  with  the  microscope  of  inflamed  areas  has 
shown  that  the  leucocytes  slip  out  of  the  capillaries  by 


FIG.  7. — A  leucocyte  is  escaping  through  a  cleft  between  adjoining 
cells  of  a  capillary  wall.  Such  a  passage  of  the  ameboid  corpuscles  from 
within  the  vessels  to  the  tissue  spaces  outside  is  called  diapedesis. 


exercising  their  power  of  ameboid  movement.  The 
walls  of  these,  the  most  slender  of  the  blood-vessels, 
are  exquisitely  thin.  The  cells  which  compose  them 
are  flattened  to  an  extreme  degree  and  where  they 
are  joined  along  their  edges  there  seems  to  be  little 
resistance  to  force  applied  to  push  them  apart.  These 
joints  are  forced  by  the  leucocytes  which  gradually 
transfer  themselves  from  the  interior  to  the  outside 
by  flowing  through  the  minute  gaps  thus  opened.  While 
the  operation  is  in  progress  the  leucocyte  which  is  being 
watched  consists  of  two  principal  masses  united  by  a 
strand  that  runs  through  the  crack.  One  of  the  masses 


CONTRACTILE    TISSUES  57 

is  increasing  and  the  other  diminishing  as  the  transfer 
goes  on. 

Ciliated  Cells. — Many  one-celled  animals  have  upon 
their  surface  a  sort  of  nap  or  pile  which  is  made  up  of 
very  fine  contractile  extensions  of  the  cell  substance. 
The  suggestion  is  of  bristles  set  in  a  brush.  Every 
individual  "  bristle" — the  term  inevitably  suggests 
something  a  great  deal  coarser  than  the  reality — is 
swinging  back  and  forth.  A  balanced,  pendular  move- 
ment of  this  kind  would  slightly  stir  the  water  nearest 
to  the  infusorium  but  could  not  materially  affect  the 
situation.  But  the  movement  of  the  cilia,  as  these  tiny 
processes  are  called,  is  not  balanced  and  pendular.  It 
has  a  most  curious  unsymmetric  character.  The  stroke 
made  in  one  direction  is  sharp  and  decisive,  the  recovery 
is  slower  and  more  gentle.  One  is  reminded  of  the 
handling  of  a  whip  or  of  an  oar;  in  either  of  these  cases  a 
forcible  stroke  in  one  direction  is  followed  by  a  less 
energetic  return. 

A  ciliated  infusorium  when  not  attached  to  any 
anchorage  is  propelled  here  and  there  by  its  waving 
cilia.  The  action  is  like  that  of  the  many  oars  of  a 
Roman  galley.  If,  however,  the  animalcule  is  fixed  in  its 
position  the  movement  brought  about  by  the  cilia  is  not 
in  the  cell  but  in  the  adjacent  water.  Currents  are 
maintained  which  are  in  many  cases  so  directed  as  to 
bring  food  particles  within  reach  of  the  infusorium.  The 
same  currents  must  assist  in  ministering  to  respiration 
by  sweeping  away  water  which  has  received  carbon  dioxid 
and  replacing  it  with  a  fresh  portion  containing  available 
oxygen. 

The  ciliated  cells  which  are  found  in  the  higher  animals 
are  usually  arranged  in  mosaic  fashion  to  form  epithelial 
surfaces.  The  cilia  are  upon  the  exposed  aspect  and 
their  beating  is  effective  in  a  direction  that  is  the  same 
for  all  the  assembled  cells.  Cilia  in  such  localities  are 
undoubtedly  overlaid  by  a  film  of  moisture  or  mucus  and 
this  is  kept  travelling  by  their  rapidly  repeated  strokes. 


58 


HUMAN    PHYSIOLOGY 


Any  small  bodies  which  adhere  to  such  surfaces  are 
accordingly  carried  along  with  the  creeping  film.  Prog- 
ress is  slow,  it  may  be  only  y2  inch  in  a  minute,  but  it 
is  definite  and  sometimes  a  matter  of  importance. 

The  effects  of  ciliary  movement  may  be  seen  when  the 
esophagus  of  a  frog  is  laid  open  and  bits  of  chalk  are 
sprinkled  upon  the  lining  membrane.  These  will  be 
taken  steadily  toward  the  stomach.  The  cilia  con- 
tinue to  be-  active  long  after  the  death  of  most  other 
tissues.  This  demonstration  upon  the  frog  is  so  com- 
monly made  that  students  need  to  be  told  that  there  are 


FIG.  8. — A  portion  of  ciliated  epithelium  as  it  might  be  seen  under 
the  microscope  if  that  instrument  could  give  a  perspective  effect. 
At  the  front  the  cells  are  sectioned,  while  above  and  receding  we  have 
their  ciliated  surface.  There  is  a  suggestion  of  coordinated  movement 
in  that  the  cilia  in  great  numbers  have  a  parallel  direction  but  it  is  not 
supposed  that  the  order  of  the  actual  performance  can  be  presented 
to  the  eye. 

no  cilia  upon  the  lining  of  the  esophagus  in  the  mammal. 
The  cleansing  of  this  passage  which  is  accomplished  by 
the  cilia  in  the  frog  is  effected  in  ourselves  by  swallowing 
saliva  or  water. 

Human  beings  do  have  cilia  in  the  respiratory  tract. 
The  lining  of  the  nasal  cavities  is  equipped  in  this  way 
and  so  is  that  of  the  lower  passages  leading  from  the 
larynx  to  the  depths  of  the  lungs.  Particles  of  dust, 
including  many  germs,  which  settle  upon  these  surfaces 
are  not  allowed  to  remain  where  they  fall  but  are  at 
once  put  in  motion.  In  the  bronchial  tubes  the  move- 


CONTRACTILE    TISSUES  59 

ment  is  from  below  upward  toward  the  throat;  there  is 
some  doubt  as  to  the  course  of  the  ciliary  currents  in  the 
nose.  Dust  which  might  accumulate  in  the  lungs  with 
serious  results  is  continually  cleared  away  and  gathered 
temporarily  about  the  root  of  the  tongue.  Eventually 
it  is  swallowed.  We  may  not  find  the  notion  agreeable, 
but  it  is  a  fact  that  all  the  people  in  a  dusty  place  are 
acting  as  vacuum  cleaners,  freeing  the  air  of  a  part  of 
the  suspended  material  and  depositing  it  in  their  own 
stomachs.  Note  that  the  direction  of  the  currents  in 
the  trachea  is  upward  while  in  the  frog's  esophagus  it  is 
downward. 


FIG.  9. — The  sea  anemone.     See  text. 

The  microscopic  air-sacs  to  which  the  finest  bronchial 
tubes  lead  are  without  cilia.  There  is  no  provision  for 
the  removal  to  the  throat  of  dust  which  runs  the  gauntlet 
without  being  arrested  and  arrives  in  these  terminal 
chambers.  Such  dust  will  remain  in  the  tissue  of  the 
lungs  and  may  discolor  those  organs  to  a  degree  which 
varies  with  the  environment  of  the  individual.  Coal 
miners  have  their  lungs  greatly  stained  with  the  dust 
they  have  inhaled,  but  the  portion  which  is  retained  must 
be  an  exceedingly  small  fraction  of  the  total  which  they 
have  breathed. 

Do  cilia  ever  reverse  the  direction  of  their  effective 
stroke?  The  question  cannot  be  answered  surely  for 
the  mammal,  but  a  reversal  has  been  observed  in  a 


60  HUMAN    PHYSIOLOGY 

lowlier  type  of  organism,  the  sea  anemone.  This 
animal  is  found  adhering  to  the  bottom  of  pools  of  salt 
water  along  rocky  coasts.  Its  appearance  is  that  of  a 
vegetable  rather  than  an  animal  type;  it  is  a  short 
cylinder  crowned  with  a  circle  of  tentacles.  Inside  the 
ring  of  tentacles  there  is  a  flat  surface  surrounding  a 
mouth.  The  zone  between  the  tentacles  and  the  mouth 
is  ciliated.  If  a  grain  of  sand  is  dropped  upon  this 
area  it  is  seen  to  move  away  from  the  oral  opening. 
Thus  the  cilia  are  ordinarily  so  acting  as  to  protect  the 
animal  against  the  tendency  to  become  filled  with  use- 
less debris.  This  is  the  more  important  because  the 
simple  digestive  cavity  of  the  sea  anemone  has  no 
outlet. 

If  the  experimenter  places  a  morsel  of  meat  instead  of  a 
sand  grain  upon  the  surface  which  he  is  observing  the 
result  is  very  curious.  The  bit  of  meat  at  first  moves  a 
little  away  from  the  mouth.  Then  it  comes  to  a  stand- 
still and  in  a  moment  more  it  is  journeying  toward  the 
mouth  into  which  it  presently  falls.  The  chemical 
composition  of  the  edible  particle  has  been  such  as  to 
modify  the  action  of  the  ciliated  cells.  The  mechanism 
of  this  reversal  might  be  pictured  in  various  ways,  but 
such  attempts  would  be  purely  speculative  and  we 
will  content  ourselves  with  the  mere  statement  of  the 
facts. 

Muscular  Tissue. — Ameboid  cells  are  found  singly  and 
the  activities  of  one  are  independent  of  those  of  another. 
Ciliated  cells,  as  seen  in  the  higher  forms,  are  in  a  fixed 
association.  The  evident  movements  of  such  forms 
are  produced  by  the  coordinated  working  of  cells  organ- 
ized into  those  tissues  which  we  speak  of  as  muscular. 
Mammalian  muscle  is  of  three  principal  varieties.  The 
kind  which  seems  distinctly  the  most  primitive  is 
that  which  we  call  plain  or  smooth  muscular  tissue. 
This  is  found  chiefly  in  the  internal  organs  and  it  is 
sometimes  termed  visceral  muscle  because  of  this  fact. 
A  unique  sort  of  muscle  is  found  in  the  walls  of  the  heart 


CONTRACTILE    TISSUES  61 

and  is  consequently  named  cardiac.  The  third  order, 
which  is  responsible  for  the  movements  of  the  limbs, 
for  breathing,  balancing,  facial  expression,  speech,  etc., 
is  best  called  skeletal. 

Smooth  Muscle. — The  properties  of  cardiac  muscle 
may  well  be  taken  up  in  connection  with  the  physiology 
of  the  heart.  Those  of  skeletal  muscle  will  be  discussed 
in  the  next  chapter.  It  will  be  desirable  at  this  place  to 
say  something  of  smooth  muscle.  We  have  just  said 
that  this  kind  of  contractile  tissue  is  most  conspicuous 
in  the  internal  organs.  For  example,  it  is  responsible 
for  the  movements  of  the  alimentary  canal,  the  con- 
tractions of  the  gall-bladder,  the  urinary  bladder,  and 
the  uterus.  But  is  not  restricted  to  these  localities. 


FIG.   10. — Cells  of  smooth  muscle. 

It  occurs  in  the  blood-vessels,  the  bronchial  tubes,  the 
eye,  and,  sparsely  distributed,  in  the  skin. 

The  cells  of  smooth  muscle  are  nowhere  massed  to  form 
layers  of  any  great  thickness.  They  are  generally 
arranged  to  form  sheets  adapted  to  enter  into  the  struc- 
ture of  hollow  organs  or  vessels.  The  individual  cells 
are  slender  and  elongated,  tapering  to  pointed  ends. 
In  each  is  a  single  nucleus.  The  terms  smooth  and  plain 
applied  to  these  cells  have  reference  to  the  contrast 
which  exists  between  them  and  the  fibers  of  skeletal 
muscle;  the  difference  will  be  apparent  later.  When 
smooth  muscle  cells  contract  each  one  shortens  and 
thickens  and,  since  the  majority  have  a  parallel  direction, 
a  corresponding  changeX  in  dimensions  may  take  place 
over  an  area  of  considerable  extent. 


62  HUMAN    PHYSIOLOGY 

Movements  brought  about  by  smooth  muscle  are  never 
very  rapid.  In  most  cases  they  can  fairly  be  called 
gradual  or  sluggish.  When  the  stomach  of  a  living 
animal  is  exposed  to  inspection  creases  are  seen  in  its 
surface  and  the  position  of  these  creases  steadily  shifts 
toward  the  adjoining  intestine,  but  the  progress  is  so  slow 
that  in  the  cat  30  seconds  may  be  occupied  in  traversing 
an  inch.  Contractions  of  smooth  muscle  are  never 
sharp  twitches;  their  onset  and  their  disappearance  are 
both  gentle  and  slow.  The  reader  will  perhaps  question 
this  as  he  recalls  the  convulsive  character  of  vomiting 
movements,  but  these  are  not  true  gastric  contractions. 
They  are  produced  by  skeletal  muscles  bearing  upon 
the  outside  of  the  stomach. 

Varieties  of  Muscular  Movements. — Muscular  move- 
ments of  all  varieties  fall  into  two  classes  according  to 
their  causation.  Some  are  due  to  the  influence  brought 
to  bear  through  the  central  nervous  system.  This  is 
true  of  those  made  by  the  skeletal  muscles.  Every 
breath  that  we  take  is  the  result  of  a  distinct  act  on  the 
part  of  a  certain  cell-group  in  the  brain.  This  is  not 
so  at  all  in  the  case  of  the  beating  heart.  Here  the 
successive  contractions  testify  to  an  independent  rhyth- 
mic tendency  resident  in  the  cardiac  muscle.  In  other 
words,  the  heart  would  continue  to  beat  even  though 
disconnected  from  the  central  nervous  system.  We  ex- 
press this  fact  by  saying  that  cardiac  muscle  is  automatic 
and  the  same  may  be  said  in  a  general  way  of  smooth 
muscle. 

A  ring  cut  from  the  stomach  of  a  frog  and  suspended 
so  that  its  contractions  shall  lift  a  light  lever  will,  under 
favorable  conditions,  shorten  and  relax  with  a  slow 
rhythm  during  many  hours.  The  same  behavior  has 
been  described  for  the  urinary  bladder  of  a  cat.  The 
automatic  property  is  an  important  matter  to  be  reck- 
oned with  in  considering  both  the  physiology  and  the 
hygiene  of  the  alimentary  tract.  One  inference  may 
be  drawn  without  delay:  namely,  that  when  we  have  to 


CONTRACTILE    TISSUES  63 

do  with  a  tissue  that  is  fundamentally  automatic,  the 
influence  of  the  nervous  system  may  be  either  to  increase 
or  to  diminish  the  degree  of  activity.  The  idea  that  the 
nervous  system  excites  action  is  familiar,  the  conception 
that  it  may  also  abate  activity  must  be  borne  in  mind. 
Such  restraint  is  called  inhibition. 

Tone. — An  organ  with  smooth  muscle  in  its  walls  may 
be  much  more  fully  relaxed  at  one  time  than  at  another. 
When  it  appears  exceptionally  large  we  need  not  con- 
clude that  it  is  forcibly  distended  by  its  contents  as  we 
should  assume  of  a  rubber  bag.  The  stomach,  for  in- 
stance, may  be  much  more  capacious  at  one  time  than 
at  another  and  yet  it  may  exert  no  more  pressure  upon 
the  food  inside  when  it  is  very  large  than  when  it  is  small. 
The  adaptations  exhibited  by  the  hollow  organs  toward 
their  varying  contents  are  referred  to  as  tone  changes. 
If  the  stomach  is  dilated  to  accommodate  a  meal  we  say 
that  its  tone  has  been  lowered.  The  student  must 
clearly  distinguish  between  such  changes  and  actual 
stretching.  Tone  may  be  defined  as  a  residue  of  con- 
traction or  incomplete  relaxation.  The  conception  is 
one  which  we  shall  constantly  be  called  upon  to  entertain. 
The  equivalent  words,  tonus  or  tonicity,  may  be  used. 

The  facts  of  tone  variation  are  well  illustrated  by  the 
behavior  of  the  urinary  bladder.  When  this  container 
has  just  been  emptied,  its  cavity  is  practically  ob- 
literated. At  another  time  it  may  hold  a  pint.  But 
everyone  knows  that  the  urgency  of  the  call  to  empty  the 
bladder  is  not  by  any  means  proportional  to  the  quantity 
of  its  contents.  The  desire  may  be  strong  when  the 
organ  is  very  small.  If  we  had  to  do  with  a  simple 
elastic  sac  the  internal  .pressure  would  necessarily  cor- 
respond with  the  degree  of  distention  but  the  walls  are 
living  tissue  and  can  contract  and  relax  independently 
of  the  amount  of  liquid  enclosed.  Tone  has  to  be  recog- 
nized also  as  an  essential  property  of  the  arteries,  es- 
pecially those  of  the  smallest  order.  Variations  of  ar- 
terial tone  will  call  for  careful  attention  at  another  time. 


64  HUMAN    PHYSIOLOGY 

Tone  changes  are  noted  in  the  heart  as  well  as  in  the 
organs  in  which  smooth  muscle  is  present.  The  heart, 
like  the  stomach  although  within  narrower  limits,  may 
be  a  larger  organ  at  one  time  than  at  another.  If 
we  make  use  of  any  method  to  record  the  beating  of  the 
heart  we  may  find  that  the  rhythmic  beats  do  not  rise 
from  a  uniform  but  from  a  variable  base-level.  This  is 
to  say  that  the  degree  of  relaxation  is  sometimes  more 
profound  than  an  average  and  sometimes  less  so.  Tone 
changes  in  skeletal  muscle  are  not  so  readily  apparent 
as  in  the  other  kinds  of  contractile  tissue  but  there  is 
no  doubt  that  they  occur.  The  consequences  are,  in 
this  case,  somewhat  different  from  those  that  have  been 
described. 

It  has  been  shown  that  the  value  of  tone  changes  such 
as  take  place  in  smooth  muscle  consists  largely  in 
adapting  hollow  viscera  to  their  varying  contents. 
The  skeletal  muscles  do  not,  as  a  rule,  bend  around 
cavities;  usually  each  one  is  arranged  to  cause  motion 
on  the  part  of  a  bone.  There  can  ordinarily  be  recog- 
nized for  each  skeletal  muscle  another,  or  perhaps 
more  than  one,  adapted  to  counteract  its  tension  and 
to  produce  an  opposite  movement.  So  we  speak  of 
antagonistic  or  opposing  muscles  in  the  skeletal  system. 

Increase  of  tone  in  skeletal  muscles  has,  as  its  main 
result,  the  establishment  of  a  greater  rigidity  and  so 
of  a  greater  resistance  to  external  forces  applied  to  cause 
motion  at  the  joints.  Where  smooth  muscle  with 
heightened  tone  tends  to  diminish  the  cavity  of  whose 
wall  it  forms  a  part,  skeletal  muscle  is  known  to  be  in 
tonic  contraction  by  the  firmness  that  is  noticed  in  the 
affected  region  and  by  its  own  relative  hardness.  Good 
posture  depends  on  proper  tone  maintained  by  many 
associated  muscles. 


CHAPTER  V 
SKELETAL  MUSCLE 

The  skeletal  muscles  are  of  various  sizes  and  shapes. 
Many  are  elongated  cylinders  or  prisms;  others  are  in 
the  form  of  comparatively  thin  sheets.  Most  of  the 
muscles  of  the  limbs  can  be  fairly  described  as  belong- 
ing to  the  first  class  while  the  other  type  is  found  in  the 
walls  of  the  abdomen.  The  longest  muscles  extend 
perhaps  as  much  as  18  inches  between  attachments, 
the  smallest  recognized  as  individuals  are  tiny  affairs  in 
the  cavity  of  the  middle  ear.  These  are  only  a  fraction 
of  an  inch  in  length. 

When  we  observe  a  muscle  in  action  we  can  usually 
recognize  that  one .  end  is  relatively  fixed  in  position 
while  the  other  is  moved  by  its  contraction.  The 
comparatively  stationary  end  is  the  origin,  the  movable 
one  is  the  insertion.  The  distinction  is  not  always  clear 
and  we  can  think  of  cases  in  which  the  two  may  seem 
to  be  interchangeable,  but  there  is  generally  obvious 
reason  to  assume  a  certain  type  of  action.  A  con- 
venient choice  for  illustrative  purposes  is  the  biceps,  on 
the  front  of  the  upper  arm.  This  is  the  muscle  most 
often  exhibited  with  pride  by  the  schoolboy. 

The  biceps  is  attached  to  the  shoulder-blade  above  and 
to  one  of  the  forearm  bones,  the  radius,  below.  It  tapers 
toward  its  extremities,  and  a  little  inspection  shows 
that  these  terminal  parts  are  not  contractile,  but  con- 
nective tissue.  They  are  tendons.  The  biceps  has  two 
tendinous  extensions  at  its  upper  end  and  takes  its 
name,  "  two-headed,"  from  this  fact.  The  intermediate 
portion  of  the  muscle  is  convex  in  contour  and  is  called 
its  belly.  This  is  the  active,  contractile  part,  but  it  is 
5  65 


66 


HUMAN    PHYSIOLOGY 


necessary  to  emphasize  the  teaching  that  even  here 
connective  tissue  is  present.  It  exists  in  a  sheath, 
enveloping  the  muscle,  and  it  also  subdivides  it  in- 
ternally, blocking  off  its  substance  into  many  bundles  in 
the  way  which  is  so  evident  in  a  beefsteak.  It  is  easy 
to  forget  that  connective  tissue  is 
everywhere  associated  with  the  con- 
tractile; it  is  in  fact  indispensable  to 
the  mechanism.  It  may  be  said  to 
constitute  a  harness  by  means  of  which 
the  innumerable  living  units  trans- 
mit their  combined  force. 

It  will  be  found  upon  examination 
that  the  moving  bones  are  levers  with 
such  relations  that  the  pull  of  the 
muscles  is  generally  upon  the  short 
arm.  It  is,  in  other  words,  nearer 
the  fulcrum  than  is  the  load  to  be 
lifted.  The  organization  is  therefore 
one  which  secures  speed  and  extent 
in  motion  at  the  expense  of  sheer 
force.  When  the  forearm  is  held 
_,  , .  horizontal  and  a  10-pound  weight  is 

FIG.    11. — The    bi-  ,.,111,1 

ceps  is  shown  with  its  upon  the  palm  of  the  hand,  the  actual 
chief  antagonist,  the  tension  maintained  by  the  biceps  and 

bendsStheTehibowCaend  one  or  two  auxiliary  muscles  must  be 
is  termed  a  flexor  more  like  100  pounds.  The  student 
muscle,  the  triceps,  may  find  himself  reluctant  to  believe 
oountrgmoave0ment,hies  this,  but  a  little  attention  to  the  pro- 
an  extensor.  portions  of  the  parts  concerned  will 

compel  him  to  assent  to  the  conclu- 
sion. The  muscles  which  bring  the  lower  jaw  against 
the  upper  are  so  attached  as  to  apply  almost  their  whole 
power  in  the  region  of  the  molars  and  a  pressure  of  270 
pounds  has  been  recorded  between  these  teeth. 

Muscle  Fibers. — Let  us  now  attend  to  the  nature  of  the 
living  units  just  mentioned.  They  are  the  fibers  of  the 
muscle.  A  single  one  has  about  the  dimensions  of  a 


SKELETAL    MUSCLE 


67 


short  piece  of  hair:  it  may  be  an  inch  long  but  only 
inch  in  diameter.  It  is  a  cylinder  and  so  long  in 
proportion  to  its  thickness  that  our  diagrams  cannot 
show  both  ends  and  still  be 
broad  enough  for  pictorial  clear- 
ness. The  fiber  is  a  modified 
cell.  Instead  of  having  a  single 
nucleus  it  has  a  considerable 
number  of  nuclei  at  intervals 
near  its  surface.  'While  most 
animal  cells  are  practically  naked 
a  muscle  fiber  has  a  well-defined 
envelope,  the  sarcolemma.  At 
either  end  of  the  fiber  the  sarco- 
lemma is  continuous  with  the 
connective  tissue.  So  it  comes 
about  that  each  fiber  is  a  muscle 
on  a  small  scale;  it  consists  of 
an  elongated  body  of  living  con- 
tractile substance  within  a  sheath 
and  attached  at  its  ends  to  non- 
contractile  tissue  through  which 
its  pull  may  be  applied. 

Skeletal  muscle  is  often  called 
striped  or  striated.  The  reference 
is  to  fine  transverse  markings 
which  one  sees  in  the  fibers  when 
they  are  highly  magnified.  The 
designation  smooth,  fixed  upon 
the  visceral  type  of  muscle  dealt 
with  in  the  preceding  chapter,  is 
used  because  these  cross  mark- 
ings are  absent  from  such  cells. 
The  striations  are  not  surface 
marks  upon  the  sarcolemma,  but  stand  for  an  obscure 
but  highly  specialized  internal  organization  of  the  living 
matter.  Very  different  interpretations  of  their  meaning 
have  been  advanced  by  different  investigators. 


FIG.  12. — Portions  of 
four  fibers  of  skeletal  mus- 
cle. If  carried  out  to  their 
ends  these  would  be  several 
feet  long  for  they  are  often 
as  much  as  500  times  longer 
than  they  are  in  diameter. 
Two  are  crossed  to  suggest 
their  relative  transparency. 
The  one  at  the  right  has 
been  injured  and  the  sar- 
colemma remains  spanning 
a  break  in  the  striated  pro- 
toplasm. 


68 


HUMAN    PHYSIOLOGY 


The  disposition  of  the  fibers  in  a  mass  of  muscle  may 
be  curiously  varied.  They  may  be  laid  parallel  through- 
out or  they  may  have  quite  another  grouping.  When  a 
muscle  has  the  character  known  as  penniform  we  can 
recognize  the  arrangement  as  possessing  the  greatest 
value.  An  example  of  a  penniform  muscle  is  the 
gastrocnemius,  the  chief  calf  muscle, 
as  it  may  be  found  in  the  frog. 
A  longitudinal  section  shows  that 
there  is  an  internal  core  of  con- 
nective tissue  which  is  really  an 
extension  of  the  upper  tendon  by 
which  connection  is  made  with  the 
thigh-bone.  From  this  core  the 
fibers  run  outward  and  downward. 
They  are  short  and  seem  to  be  in- 
serted in  the  superficial  envelope 
of  the  muscle. 

When  the  gastrocnemius  con- 
tracts, the  external  layer  is  drawn 
up  and  the  lower  tendon  (tendon 
of  Achilles)  is  lifted  with  it.  The 
central  connective  tissue  is  motion- 
less. The  core  furnishes  a  place  of 
origin  for  a  vastly  greater  number 
of  fibers  than  could  be  accomo- 


pull  in  the  second  case 
is  that  of  a  far  greater 
number  of  fibers. 


FIG.  13.— A  band  of 
parallel  fibers  compared 
with  a  penniform  arrange- 
ment. The  former  can 
lift  through  a  longer  in- 
terval, the  latter  with 

much  more  force.  The  dated,  if  they  ran  parallel  from  the 
knee  to  the  heel.  The  power  of  the 
muscle  is  correspondingly  increased. 
At  the  same  time  its  range  of  move- 
ment as  measured  at  the  lower  end  is  much  reduced.  In 
effect,  a  relatively  long  bundle  of  muscular  tissue,  adapted 
by  its  shape  to  have  a  place  in  the  leg,  is  given  the  prop- 
erties of  a  short  muscle  of  great  thickness.  (Fig.  13.) 

Muscle  Contraction. — Muscles  of  the  order  which  we 
are  now  considering  are  not  automatic.  On  the  contrary 
they  are  thrown  into  contraction  because  they  are  con- 
nected with  the  central  nervous  system  which  presides 


SKELETAL   MUSCLE 


69 


over  them.  Break  the  connection  just  mentioned  and 
the  muscle  is  paralyzed.  For  experimental  purposes 
we  may  substitute  artificial  stimuli  for  the  normal  ones 
which  proceed  from  the  brain  and  cord.  Electric 
shocks  are  generally  employed  in  such  trials.  They  are 
preferable  for  several  reasons  to  other  kinds  of  stimuli. 
They  are  relatively  harmless  to  the  tissue  and  they  can 
be  accurately  graded  in  intensity. 

To  study  the  nature  of  muscle  contraction  an  isolated 


dr 


FIG.  14. — m  is  the  muscle  held  in  a  clamp  and  connected  with  a 
weighted  lever  the  point  of  which  bears  on  the  smoked  drum,  dr.  n 
is  the  nerve  carried  over  electrodes  through  which  it  can  be  stimulated. 
Stimulation  may  be  applied  directly  to  the  muscle. 


muscle  from  a  frog's  leg  is  usually  chosen.  The  most 
suitable  for  most  uses  is  the  gastrocnemius  since  it  is 
readily  separated,  with  little  or  no  injury,  from  the 
neighboring  structures.  It  is  left  attached  above  to  a 
bit  of  the  thigh-bone  which  is  held  in  a  clamp.  The 
tendon  below  is  made  fast  to  a  lever  which  the  muscle 
will  lift  when  it  contracts.  The  lever  is  designed  to 
reproduce  the  movements  of  the  tendon  upon  a  larger 
scale.  At  its  free  end  it  is  tipped  with  a  point  of  paper 


70  HUMAN    PHYSIOLOGY 

and  when  this  is  made  to  bear  upon  a  smoked  surface, 
either  at  rest  or  in  motion,  a  record  of  the  rise  and  fall  is 
traced.  This  is  an  example  of  what  is  called  the  graphic 
method  in  physiology. 

Suppose,  now,  that  a  muscle  has  been  prepared  and 
suspended  above  a  writing  lever.  Suitable  contacts  are 
provided  for  the  passage  of  electric  currents  through  the 
tissue.  The  experimenter  administers  a  momentary 
shock.  The  muscle  is  seen  to  twitch,  the  lever  springs 
upward  and  falls  again.  All  that  can  be  ascertained 
by  direct  observation  is  that  we  have  to  do  with  a  fairly 
quick  and  brief  response.  To  learn  more  than  this  we 


Fio.  15. — Curve  of  simple  muscular  contraction.     (HoweU.) 

must  have  recourse  to  additional  apparatus.  The 
smoked  paper  may  be  borne  upon  a  metal  drum  turned 
by  clockwork.  If  the  drum  is  revolved  at  a  brisk  rate 
while  the  muscle  is  made  to  repeat  its  sharp  twitch  a 
curve  will  be  left  which  can  be  analyzed  with  profit. 

The  curve  traced  upon  a  moving  surface  when  the 
muscle  acts  in  response  to  a  single  stimulus  is  the  curve 
of  a  "  simple  muscle  contraction."  The  speed  of  the 
recording  surface  influences  its  appearance  but  not  its 
significance.  It  will  be  found  to  have  a  definite  peak 
rather  than  a  flat  summit.  On  the  average  the  descend- 
ing slope  of  the  curve — the  record  of  the  relaxation — 
will  be  somewhat  longer  than  the  ascending  part  which 


SKELETAL    MUSCLE  71 

indicates  the  shortening  of  the  muscle.  Occasionally 
the  reverse  is  the  case.  To  translate  the  curve  and  its 
subdivisions  into  terms  of  actual  time  it  is  only  necessary 
to  have  a  tuning  fork  with  a  known  rate  of  vibration 
which  leaves  its  wavy  tracing  upon  the  drum  at  the  same 
time  that  the  muscle  is  recording. 

If  our  tuning  fork  makes  100  vibrations  per  second,  it 
is  likely  that  the  curve  described  by  the  lever  (moved  by 
a  fresh  muscle  to  which  one  stimulus  has  been  given) 
will  stretch  over  about  10  of  the  small  waves.  The 
conclusion  is  that  a  simple  contraction  can  be  executed  in 
Ko  second.  If  this  is  true  10  stimuli,  with  uniform 
intervals,  might  be  given  in  a  second  with  the  result  that 
the  muscle  would  contract  10  times  and  drop  the  lever 
to  the  base-line  after  each  contraction.  As  a  matter  of 
fact  some  fusion  would  probably  occur  because,  in  a  series 
of  repeated  simple  contractions,  the  duration  of  individual 
ones  tends  to  increase.  Human  muscle  shows  little  or 
no  superiority  to  the  frog's  in  its  capacity  for  rapidly 
repeated  movements.  The  best  that  the  trained  pianist 
or  typewriter  can  do  with  one  finger  is  to  press  a  key  10 
or  11  times  in  a  second.  We  are  far  inferior  to  flying 
insects  whose  wings  may  beat  300  times  or  more  in  the 
same  brief  interval. 

Summation. — If  a  second  stimulus  takes  effect  upon  a 
muscle  which  has  not  completed  its  relaxation,  after 
answering  to  a  first,  it  may  spring  from  its  partially 
contracted  condition  to  jDerform  a  fresh  act.  In  the 
tracing  the  second  curve  seems  to  be  mounted  upon  the 
first  and  reaches  a  greater  height.  The  second  stimulus 
may  be  so  timed  as  to  make  the  later  elevation  rise  from 
the  very  peak  of  the  earlier  one.  Two  stimuli  of  mod- 
erate intensity  may  be  more  efficient  in  forcing  up  the 
height  of  contraction  than  one  stimulus,  however  strong, 
can  possibly  be.  At  least  this  is  true  when  a  muscle 
raises  any  considerable  weight. 

Tetanic  Contractions. — As  it  is  possible  to  follow  one 
stimulus  with  another  after  such  a  short  interval  that 


72 


HUMAN   PHYSIOLOGY 


FIG.  16. — Analysis  of  tetanus.  Experiment  made  upon  the  gastroc- 
nemius  muscle  of  a  frog  to  show  that  by  increasing  the  rate  of  stimu- 
lation the  contractions,  at  first  separate  (1),  fuse  more  and  more  through 
a  series  of  incomplete  tetani  (2,  3,  4)  into  a  complete  tetanus  (5)  in  f 
which  there  is  no  indication,  so  far  as  the  record  goes,  of  a  separate 
effect  for  each  stimulus.  (Howell.)  • 


SKELETAL    MUSCLE  73 

the  muscle  does  not  have  time  to  relax,  so  a  series  of 
stimuli  can  be  given  at  so  rapid  a  rate  that  the  muscle 
is  held  throughout  at  the  height  of  contraction.  Shocks 
sent  in  with  a  frequency  of  25  in  a  second  should  suffice 
to  secure  this  result.  The  tracing  obtained  from  a 
muscle  thus  treated  has  a  plateau  character  and  the 
temptation  is  to  describe  such  a  contraction  as  con- 
tinuous. But  it  is  not  to  be  thought  of  in  this  way. 
The  seeming  continuity  of  the  process  is  the  result  of 
successive  physiologic  acts  of  contraction  permitting  no 
apparent  relaxation  to  take  place.  Sustained  con- 
tractions are  common  enough  in  our  experience;  in  fact, 
they  are  the  rule  rather  than  the  exception.  Even  our 
briefer  voluntary  movements  are  believed  to  be  produced 
by  stimuli  which  issue  in  series  rather  than  singly  from 
the  nervous  system. 

A  tetanic  contraction  is,  therefore,  the  response  of  a 
muscle  to  rapidly  repeated  stimuli  and  is  contrasted 
with  the  single  contraction  which  is  the  response  to  a 
solitary  stimulus.  The  name  tetanus,  in  medical  litera- 
ture, means  lock-jaw,  and  since  in  that  disease  the  chief 
symptom  is  the  recurrence  of  muscular  spasms  the  term 
used  by  the  physiologist  is  seen  to  be  appropriate.  A 
tetanus  may  be  either  complete  or  incomplete.  In 
the  second  case  relaxation  begins  after  each  upthrust  but 
is  interrupted  the  next  instant.  A  jagged  record  results 
with  the  experimental  preparation  and  the  contraction 
is  obviously  of  a  tremulous  or  fluttering  character. 
Strong  voluntary  contractions  are  attended  by  notice- 
able tremor.  This  has  been  attributed  to  incomplete- 
ness of  tetanus  in  the  fibers  at  work,  but  it  may  also  be 
due  to  varying  distribution  of  activity  among  different 
portions  of  the  muscles. 

Muscular  Fatigue. — If  a  frog's  muscle  is  compelled 
by  stimulation  to  make  a  long  series  of  simple  contrac- 
tions, several  points  of  interest  will  apear  in  the  record. 
It  often  happens  that  the  first  few  contractions  are 
"seen  to  have  gained  progressively  in  height.  This 


74  HUMAN    PHYSIOLOGY 

indicates  that  the  capacity  to  respond  to  stimuli  is  not 
at  its  best  when  the  preparation  is  absolutely  fresh  but 
rather  after  it  has  done  some  work.  We  have  here  an 
instance  of  "warming  up"  although  it  may  not  be 
closely  comparable  with  the  experience  for  which  that 
term  is  used.  The  word  staircase,  or  its  German  equiva- 
lent Treppe,  is  applied  to  the  early  contractions  which 
show  this  gain  in  height.  The  Treppe  is  usually  limited 
to  a  small  number  of  twitches;  a  maximum  is  then 
reached  and  for  a  time  the  contractions  are  nearly  of 
uniform  extent. 

Presently,  however,  the  height  of  the  tracings  begins 
to  denote  a  slow  decline.  We  say  that  the  muscle  has 
begun  to  exhibit  fatigue.  If  we  continue  stimulating 
at  regular  intervals  a  time  will  come  when  no  move- 
ment can  be  evoked,  but  a  vigorous  muscle  will  make  more 
than  1000  simple  contractions  before  this  limit  is  reached. 
If  one  of  these  later  and  reduced  contractions  is  traced 
over  a  tuning-fork  record,  it  is  found  to  be  prolonged  as 
well  as  low.  Closer  observation  shows  that  the  short- 
ening has  taken  place  almost  as  rapidly  as  in  an  un- 
fatigued  muscle,  but  that  the  period  of  the  relaxation  has 
been  much  lengthened.  The  frog's  muscle  when  fatigued 
is  accordingly  said  to  show  "contracture,"  that  is,  de- 
layed relaxation. 

What  has  happened  in  a  muscle  which  has  been  forced 
to  work  until  quite  fatigued?  To  answer  this  question 
we  must  consider  the  source  of  the  energy  which  has  been 
set  free.  The  general  truth  to  be  borne  in  mind  is  that 
all  such  energy  must  be  referred  to  chemical  decom- 
position. A  muscle  has  often  been  compared  with  a 
steam  engine  in  which  fuel  is  burned,  heat  produced,  and 
a  share  of  the  heat  transformed  to  do  mechanical  work. 
Recent  studies  have  served  to  show  that  the  parallel  is 
not  so  close  as  had  been  assumed.  Still  it  has  its  value. 

A. muscle  removed  from  the  body  can  receive  nothing 
of  importance  from  its  environment  except  oxygen. 
Its  fuel  supply  is  strictly  limited.  As  it  works  this 


SKELETAL    MUSCLE  75 

supply  will  be  drawn  upon  and  exhaustion  will  steadily 
come  nearer.  The  situation  is  like  that  of  an  automobile 
on  the  road.  Its  possible  travel  is  determined  by  the 
quantity  of  gasoline  carried.  The  isolated  muscle  can- 
not be  furnished  with  fresh  fuel  as  the  motor  car  can  be. 
The  comparison  suggests  that  a  fatigued  muscle  is  one 
in  which  the  store  of  fuel  is  running  short.  We  shall  find, 
however,  that  this  is  not  the  whole  story. 

A  fire  may  go  out  from  lack  of  draught  as  well  as  from 
want  of  fuel.  The  chemical  processes  in  a  muscle  which 
are  necessary  to  cause  contraction  may  be  retarded  in 
an  analogous  way.  A  deficient  oxygen  supply  will 
certainly  hasten  fatigue.  Another  condition  to  be  given 
due  weight  is  the  possible  gathering  in  the  tissue  of 
products  generated  in  the  course  of  its  own  activity. 
The  draught  provided  for  a  fire  has  a  double  purpose. 
It  introduces  oxygen  and  the  next  instant  it  sweeps 
away  the  gases  formed  in  combustion.  The  blood  which 
is  normally  flowing  through  a  muscle  has  a  like  service. 
Interrupt  the  circulation  and  the  oxygen  supply  is  cut 
off  at  the  same  time  that  provision  for  removing  carbon 
dioxid  is  abolished. 

Without  detailing  many  experiments  that  have  thrown 
light  upon  this  matter  we  may  say  that  muscular  fatigue, ' 
as  observed  in  laboratory  conditions,  is  much  more 
likely  to  be  due  to  accumulating  waste-products  than 
to  imminent  exhaustion  of  fuel.  Carbon  dioxid  is  net 
the  only  one  concerned;  another  which  receives  much 
attention  by  writers  on  the  subject  is  a  variety  of  acid 
(sarcolactic  acid)  closely  resembling  that  which  is  formed 
in  the  familiar  souring  of  milk.  This  is  certainly  pro- 
duced in  active  muscles  whenever  their  working  is  out 
of  proportion  to  the  available  oxygen  and  it  definitely 
lowers  their  capacity  for  contraction.  Still  other 
"fatigue  substances"  beside  carbon  dioxid  and  lactic 
acid  have  been  mentioned  by  different  investigators. 

If  a  muscle  is  in  the  body  instead  of  isolated  from  it, 
the  resistance  to  fatigue  will  naturally  be  increased. 


76  HUMAN    PHYSIOLOGY 

The  blood,  as  it  sweeps  through  the  small  vessels  of  the 
mass,  enriches  the  lymph  with  oxygen  and  the  fibers 
receive  the  oxygen  from  the  lymph  that  bathes  them. 
At  the  same  time  the  blood  is  relieving  the  lymph  of 
carbon  dioxid  which  the  fibers  have  unloaded  upon  it. 
In  a  somewhat  indirect  way  the  oxygen  brought  to  the 
scene  by  the  blood  disposes  of  the  sarcolactic  acid.1 
Finally,  the  blood  ministers  to  the  muscle  and  defers 
fatigue  by  bringing  new  offerings  of  food,  especially 
sugar. 

It  is  easy  to  conceive  of  a  state  of  affairs  such  that  a 
balance  may  exist  between  consumption  and  renewal. 
If,  in  addition,  the  removal  of  waste-products  is  adequate 
a  certain  average  degree  of  activity  may  be  indefinitely 
continued  without  inducing  fatigue.  This  is  actually 
realized  in  the  case  of  the  heart  which  begins  to  beat 
very  early  in  embryonic  life  and  goes  on,  it  may  be,  for 
seventy  years.  We  must  infer  that  the  decomposition 
entailed  by  every  beat  is  perfectly  compensated  before 
the  next  contraction  is  begun.  Some  of  the  muscles  used 
in  breathing  show  a  similar  ability  to  go  on  with  serial 
movements,  though  there  is  more  variation  in  their 
performance  than  is  true  of  the  heart. 

In  regard  to  the  Treppe  an  interesting  discovery  has 
been  made.  It  is  to  the  effect  that  the  gain  in  power 
during  the  early  period  of  activity  is  due  to  minute 
quantities  of  the  same  substances  which  we  call  fatigue 
products.  A  little  carbon  dioxid  is  favorable  to  the 
working  capacity  of  a  muscle  although  any  considerable 
increase  of  the  same  compound  will  depress  it.  The 
same  can  be  said  of  sarcolactic  acid  and  probably  of  still 
other  bodies.  The  idea  that  one  and  the  same  sub- 
stance may  be  classed  as  a  stimulant  or  as  a  depressant 
according  to  its  concentration  is  one  which  is  of  much 
value  as  we  consider  questions  of  hygiene. 

In  the  laboratory  we  can  study  true  muscular  fatigue, 

1  The  acid  is  not,  normally,  oxidized  but  is  forced  back  into  a  chem- 
ical compound  like  that  which  lately  gave  rise  to  it. 


SKELETAL   MUSCLE  77 

eliminating  other  elements  from  our  problem.  It  is 
important  to  point  out  that  we  cannot  simplify  the 
conditions  to  the  same  extent  when  we  deal  with  the 
living  body.  The  fatigue  which  we  know  by  experience 
cannot  be  assigned  wholly  to  the  muscles.  We  do  not 
use  these  in  everyday  life  without  using  a  large  part  of 
the  nervous  system  at  the  same  time.  We  have,  there- 
fore, to  admit  that  the  flagging  of  our  own  powers  as  we 
tire  of  any  exercise  may  be  owing  to  a  decline  of  efficiency 
on  the  part  of  the  nervous  elements.  How  many  factors 
we  ought  to  take  into  account  can  hardly  be  estimated  at 
this  time  but  will  be  more  clear  later.  "A  chain  is  no 
stronger  than  its  weakest  link"  and  endurance  in  action 
will  be  limited  by  the  failure  of  any  one  of  the  several 
mechanisms  which  are  in  cooperation. 

The  Energy  Transformations  in  Skeletal  Muscle. — 
This  is  a  difficult  matter  and  we  cannot  pursue  it  far  but 
it  would  not  be  wise  to  omit  all  discussion  of  it.  We 
know  that  oxidation  goes  on  and  that  motion  results 
but  it  has  proved  exceedingly  hard  to  ascertain  what 
intermediate  processes  serve  to  connect  the  two.  We 
have  hinted  that  the  comparison  between  a  muscle  and 
a  steam  engine  is  quite  faulty  and  we  must  now  show  in 
what  chief  respect.  In  the  locomotive  oxidation  yields 
heat  and  a  part  of  the  heat  energy  is  presently  trans- 
formed to  work.  Heat  arises  in  working  muscles  also 
and  is  more  or  less  proportional  to  the  intensity  of  their 
action.  The  temptation  was  to  assume  that  in  this  case, 
as  in  the  other,  all  the  energy  set  free  existed  at  first  as 
heat  and  that  a  fraction  of  it,  an  instant  later,  was 
applied  to  the  performance  of  work.  The  earlier  theories 
were  designed  to  show  how  heat  might  be  converted  into 
movement  by  the  muscle. 

Recent  studies  in  which  methods  of  extraordinary 
refinement  have  been  used  seem  to  prove  that  most  of 
the  heat  developed  in  connection  with  the  execution  of 
a  simple  contraction  arises  after  the  movement.  The 
inference  is  that  the  oxidation  does  not  immediately 


78  HUMAN    PHYSIOLOGY 

precede  contraction  but  comes  in  its  wake.  Yet  we 
believe  as  firmly  as  ever  in  the  conservation  of  energy 
and  that  there  is  a  correspondence  between  the  work 
done  and  the  fuel  consumed.  Are  there  any  analogies 
which  can  be  employed  now  that  the  steam  engine  has 
failed  us?  We  shall  find  that  there  are. 

If  oxidation  follows  muscular  movement  we  must 
conclude  that  its  purpose  is  to  make  ready  for  subsequent 
action.  Some  of  the  energy  liberated  must  become 
latent  in  some  way  and  so  be  held  ready  for  release  either 
shortly  or  after  some  time.  This  is  not  an  unfamiliar 
condition  in  the  field  of  mechanics.  In  the  pile  driver, 
for  instance,  the  engine  works  to  lift  the  weight  which 
may  then  be  hung  at  the  top  of  the  guides  and  let  fall 
whenever  desired.  The  oxidation  has  taken  place  after 
the  fall  of  the  weight  upon  the  pile.  To  be  sure,  it  has 
to  precede  the  next  descent,  but  an  indefinite  time  may 
pass  before  this  takes  place.  So,  in  the  case  of  the  air 
brake,  preparation  is  made  by  pumping  air  into  con- 
tainers, but  this  may  be  done  long  before  the  occasion 
for  using  the  brake.  When  it  is  set  there  is  no  call  for 
the  sudden  burning  of  fuel,  but  directly  after  the  train 
has  been  stopped  we  hear  the  pump  at  work,  preparing 
the  system  for  later  service. 

The  muscle  may  also  be  compared  with  an  electric 
power  house  in  which  there  are  storage  batteries.  The 
engines  are  run  when  convenient  to  drive  the  dynamos 
and  charge  the  batteries.  These  will  give  back  energy 
at  any  time  in  connection  with  chemical  changes  in  their 
cells.  It  will  be  found  that  this  is  quite  closely  analogous 
to  the  case  of  the  muscle.  The  simplest  image  that  we 
can  select  is  that  of  the  bent  bow,  drawn  back  and  held 
in  readiness  for  letting  go.  We  must  remember  that 
when  a  muscle  is  in  tetanus  the  oxidation  that  follows 
each  momentary  act  of  contraction  is  so  closely  followed 
by  another  movement  that  the  order  of  the  processes  is 
obscured.  The  fibers  of  such  a  muscle  are  like  a  legion 
of  archers,  discharging  arrows  as  fast  as  they  can  handle 


SKELETAL    MUSCLE  79 

their  weapons.  The  spectator  would  see  the  flight  of 
the  arrows  and  might  not  pause  to  reflect  that  the  force 
applied  by  the  marksmen  is  opposite  in  direction  and 
imparted  in  the  intervals  between  the  volleys.  The 
period  of  relaxation  is  not  marked  by  a  suspension  of 
activity  on  the  part  of  a  muscle  but  is  occupied  by  a 
most  important  readjustment. 

The  Economy  of  the  Contractile  Process. — Calcu- 
lations are  often  made  with  reference  to  steam  engines 
with  the  object  of  discovering  how  great  a  part  of  the 
total  energy  represented  by  the  coal  can  be  converted 
to  actual  horse-power  or  work.  It  has  not  been  possible 
to  raise  the  proportion  thus  convertible  much  above  15 
per  cent.  As  a  rule  something  like  seven-eighths  of  the 
energy  liberated  when  the  fuel  is  burned  escapes  the 
application  which  the  engineer  would  like  to  make  of 
it.  Most  of  it  goes  up  the  chimney  as  heat.  Similar 
computations  can  be  made  for  the  skeletal  muscle  for 
here,  too,  we  have  a  mechanism  in  which  fuel  is  con- 
sumed and  heat  and  work  produced. 

Under  the  best  conditions  a  muscle  can  make  a  much 
better  showing  in  this  respect  than  the  engine.  A 
record  of  about  50  per  cent,  has  been  attained  with 
simple  contractions.  With  tetanic  contractions  the 
efficiency  is  lower  and  often  of  the  same  order  as  that  of 
the  engine.  It  is  not  practicable  in  a  work  like  this  to 
detail  the  ingenious  methods  by  which  these  facts  have 
been  established.  A  man  going  up  a  mountain  is  prob- 
ably forced  to  radiate  to  his  environment  and  make 
latent  by  evaporation  as  much  as  three  times  the  heat 
which  would  be  equivalent  to  the  measurable  work 
accomplished. 

Skeletal  Muscles  as  Heat-producing  Organs. — The 
production  of  heat  we  have  been  discussing  is  not  to  be 
thought  of  as  an  absolute  and  unfortunate  waste. 
Human  beings  and  warm-blooded  animals  usually  have 
to  maintain  body  temperatures  many  degrees  higher 
than  those  of  their  surroundings.  Such  a  condition  can 


80  HUMAN    PHYSIOLOGY 

be  kept  up  only  by  the  generation  of  heat  in  their  bodies 
and  this  evolution  goes  on  chiefly  in  the  skeletal  muscles. 
A  contribution  to  the  total  is  also  made  by  the  heart 
and  the  glands.  When  the  outside  temperature  equals 
or  exceeds  that  of  the  body  the  unavoidable  continuance 
of  internal  heat  production  does  add  to  the  difficulties 
of  the  situation.  On  the  other  hand,  an  animal  whose 
muscles  have  been  removed  from  the  control  of  the 
nervous  system,  so  that  they  cannot  be  called  into  play 
on  occasion,  has  little  resistance  to  cold.  We  save 
ourselves  from  freezing  to  death — partly,  to  be  sure,  by 
extra  clothing — but  largely  because  when  we  feel  cold 
we  are  instinctively  active.  If  we  try  not  to  be  so  we  soon 
shiver  or  grow  tense  in  the  attempt  to  remain  still. 
Either  shivering  or  tension  means  muscle  contraction; 
so  nature  refuses  to  be  cheated. 


CHAPTER  VI 


SKELETAL  MUSCLE  AND  THE  NERVOUS  SYSTEM 

The  statement  has  been  made  that  the  contractile 
tissue  of  the  heart  and  also  that  which  we  call  smooth 
muscle  may  be  considered  " automatic" — that  is,  having 
an  inherent  tendency  to  keep  contracting  and  relaxing. 
These  tissues  are  subject  to  ner- 
vous regulation  which  may  either 
augment  or  repress  the  native  prop- 
erty. Skeletal  muscle  has  been 
said  not  to  be  automatic.  All 
its  movements  in  life  are  caused 
through  the  central  nervous  system 
and  we  must  now  give  our  atten- 
tion to  this  relationship.  It  is  one 
which  holds  as  truly  for  the  single 
muscle  fiber  as  for  the  multiple  ar- 
rangement. 

Each  muscle  fiber  is  believed  to       FlG<  17>_The  segment 
have  one  and  only  one   point   at    is  from  a  fiber  of  skeletal 

which   the   Stimulus   from   the  ner-     muscle    near  its  middle. 

vous  system  affects  it.  This  is 
about  its  middle.  A  theoretic  ad- 
vantage may  be  connected  with 
this  position:  the  whole  length  of  motor'end-plate  (e.P.). 
the  fiber  will  be  more  quickly  in- 
volved if.it  is  " touched  off"  at  an  intermediate  point 
than  it  would  be  if  it  were  excited  at  one  end.  The  dis- 
turbance, spreading  in  two  directions  along  the  fiber, 
will  traverse  it  in  less  time  than  if  it  ran  from  one  ex- 
tremity to  the  other.  However,  the  saving  of  time  is 
less  than  would  be  supposed  and  is  perhaps  of  no  prac- 
tical significance.  The  point  at  which  the  stimulus  is 

6  81 


The  motor  nerve,  fiber 
(nf)  comes  into  relation 
with  the  muscle  proto- 
plasm through  the  pecu- 
liar junctionknown  as  the 


82  HUMAN    PHYSIOLOGY 

imparted  is  known  as  the  motor  end-plate.  It  is  the  junc- 
tion of  a  nerve  fiber  with  a  muscle  fiber. 

From  what  has  been  said  it  will  appear  that  every 
muscle  fiber  is  reached  by  a  nerve  fiber.  These  nerve 
fibers,  individually  too  small  to  be  visible,  come  to  a 
given  muscle  in  its  motor  nerve  which  contains  so  many 
of  them  that  it  is  a  cord  of  some  size.  We  must  not  con- 
clude that  a  muscle  having  100,000  fibers  will  receive  an 
equal  number  of  nerve  fibers  from  without.  The  nerve 
fibers  branch  freely  before  uniting  with  the  contractile 
units  and  the  result  is  that  from  10  to  100  muscle  fibers 
may  be  governed  through  a  single  fiber  such  as  exists  in 
the  nerve  outside  the  muscle.  If  this  were  not  so, 
nerves  would  be  much  larger  than  they  are  in  reality. 

Nerve  Fibers. — A  nerve  fiber  is  more  slender  than  a 
muscle  fiber  and  it  is  likely  to  be  a  great  deal  longer. 
Moreover  it  is  a  compound  affair  and  cannot  be  regarded 
as  a  modified  cell.  At  first  glance  and  ever  after  it 
suggests  an  insulated  wire.  There  is  a  central  core,  the 
axon,  and  there  is  no  doubt  that  here,  as  with  the  wire, 
it  is  this  internal  core  which  is  the  essential  conductor. 
Around  the  axon  there  is  a  sheath  of  fatty  substance. 
External  to  this  again  there  is  an  extremely  thin  outer 
sheath,  the  neurilemma.  The  axon  is  continuous  through- 
out the  longest  fiber  but  the  fatty  sheath  is  a  jointed 
structure  with  interruptions  of  which  there  are  about 
twenty-five  to  the  inch.  These  interruptions  are  known 
as  Nodes  of  Ranvier. 

One  must  distinguish  clearly  between  a  nerve  fiber 
and  a  nerve.  The  nerve  is  a  cable  and  may  have 
bound  up  in  it  many  thousand  fibers.  Nerves  are 
traced  by  the  dissector  and  with  the  unaided  eye;  fibers 
require  high  powers  of  the  microscope  to  reveal  their 
character  and  numbers.  For  the  present  our  interest  is 
in  such  a  fiber  as  leads  to  a  moderate  number  of  end- 
plates  in  a  skeletal  muscle.  The  axon  of  this  fiber  is  a 
conductor  of  something  to  the  dependent  muscle  fibers, 
but  what  is  it  that  it  conveys?  How  is  the  stimulation 


SKELETAL  MUSCLE  AND  THE  NERVOUS  SYSTEM       83 

made  effective?  These  are  questions  that  we  cannot 
answer  very  fully.  It  will  be  helpful  to  begin  by  ex- 
cluding certain  conceptions. 

Nature  of  the  Nerve-impulse. — First  of  all,  that  which 
passes  along  fibers  of  the  nervous  system  is  not  matter  but 
energy.  We  do  not  have  to  do  here,  as  in  the  blood 
system,  with  tubular  conduits.  An  early  and  crude 
notion  was  to  the  effect  that  fluid  pulses  travel  along 
the  nerves.  The  idea  was  quite  elaborately  developed 
and  had  its  value  but  only  as  a  provisional  symbol. 
We  must  likewise  avoid  thinking  of  nerves  as  though 
they  were  cords  to  be  pulled  upon.  The  expressions 
" nerve  strain"  and  "nerve  tension"  encourage  this 
view  but  they  are  figurative.  A  man  is  said  to  have 
tense  nerves  when  the  tension  is  really  in  his  muscles. 
Under  such  circumstances  the  muscular  state  manifests 
a  corresponding  nervous  condition  but  is  not  to  be 
identified  with  it. 

Another  temptation  is  to  think  of  the  nerves  as  con- 
ductors of  electricity.  There  is,  in  fact,  nothing  of 
human  construction  so  suggestive  of  the  nervous  system 
as  is  the  telephone  equipment  of  a  community.  But  it  is 
generally  held  that  the  form  of  energy  which  nerves 
transmit  cannot  be  the  electric  current  as  ordinarily 
understood.  It  does  have  electric  accompaniments  and 
it  has  lately  been  urged  that  the  impulse  itself  is  essen- 
tially electric.  If  this  is  true,  however,  the  conditions 
prevailing  in  nerve  fibers  are  still  so  peculiar  as  to  forbid 
close  comparison  between  them  and  telephone  wires. 
A  wire  can  be  cut  and  spliced,  when  it  will  conduct 
nearly  as  well  as  before.  A  nerve  cannot  be  reunited 
after  cutting  so  as  to  resume  its  service. 

The  velocity  at  which  the  impulse  passes  along  the 
nerve  has  been  measured  quite  accurately.  By  suitable 
apparatus  it  is  possible  to  show  that  when  a  considerable 
length  of  nerve  is  used  to  excite  contraction  in  a  muscle 
the  response  is  distinctly  less  prompt  than  when  the 
stimulus  is  applied  close  to  the  muscle.  The  difference 


84  HUMAN    PHYSIOLOGY 

in  time  between  the  two  trials  must  be  attributed  to  the 
employment  of  an  extra  length  of  nerve  in  the  first  case. 
By  a  simple  calculation  it  appears  that  the  rate  of  trans- 
mission in  the  nerve  of  a  frog  at  room  temperature  is  in 
the  neighborhood  of  100  feet  per  second.  This  is  the 
speed  of  an  express  train  running  70  miles  an  hour. 
Higher  rates  seem  to  be  reached  in  the  warm-blooded 
animals  but  the  maximum  is  less  than  the  velocity  of 
sound  waves  in  the  air,  and  is  insignificant  by  com- 
parison with  the  speed  of  light. 

The  term  nerve-impulse  which  we  usually  use  to  denote 
the  energy  carried  along  a  nerve  does  not  bind  us  to  any 
particular  theory  as  to  its  nature.  Whatever  it  is,  it  is 
rapid  enough  to  insure  prompt  reactions  in  animals  of 
ordinary  size.  Furthermore,  it  is  not  appreciably  ex- 
hausting to  the  fibers  concerned  in  forwarding  it. 
Muscles  fatigue  with  use,  as  we  have  seen;  nerves  seem 
nearly  if  not  quite  proof  against  fatigue.  The  reader 
should  recognize  that  this  statement  is  limited  to  the 
fibers  on  which  our  attention  for  the  present  is  fixed. 
There  are  other  elements  in  the  nervous  system  which 
probably  suffer  wear  and  tear  when  in  use.  The  sus- 
ceptibility of  these  structures  to  fatigue  will  be  dis- 
cussed at  another  time. 

Origin  of  Nerve -impulses. — We  can  now  proceed  to 
investigate  the  place  of  origin  of  the  nerve-impulses 
by  which  muscles  are  thrown  into  action.  We  have 
pictured  a  cluster  of  muscle 'fibers  whose  end-plates 
are  supplied  by  the  branches  of  a  single  nerve  fiber. 
Whence  does  this  fiber 'come?  The  answer  will  depend 
upon  the  position  ojf  the  muscle  in  the  case  so  we  will 
assume  a  definite  example.  Suppose  that  it  is  the  biceps 
of  the  arm.  A  motor  fiber  ending  in  this  muscle  could 
be  traced  back  to  the  intricate  commingling  of  nerves 
in  the  neck  to  which  we  give  the  name  of  the  cervical 
plexus.  The  fiber  could,  theoretically  at  least,  be 
shown  to  have  come  by  an  unbroken  course  through 


SKELETAL  MUSCLE   AND   THE   NERVOUS  SYSTEM       85 

this  maze  and  to  have  originated  in  the  spinal  cord  near 
the  same  level. 

If  we  had  chosen  a  muscle  of  the  head — for  instance, 
the  masse ter  which  acts  upon  the  lower  jaw — we  should 
have  found  the  selected  fiber  to  have  come  through  one 
of  the  openings  of  the  skull  and  to  have  arisen  in  the 
lower  part  of  the  brain.  The  motor  fibers  for  the  leg 
muscles  come  from  the  lower  levels  of  the  spinal  cord. 
Since  all  the  skeletal  muscles  excepting  those  of  the 
head  derive  their  motor  supply  from  the  spinal  cord 
we  must  attend  now  to  some  of  the  features  of  this 
part  of  the  nervous  axis. 

The  Spinal  Cord  and  the  Spinal  Nerves. — It  has 
been  previously  pointed  out  that  the  spinal  cord  occu- 
pies a  canal  formed  by  the  arches  of  the  vertebrae. 
The  cord  is  continuous  with  the  brain  above,  a  large 
opening  in  the  base  of  the  skull  providing  for  the  union. 
Protective  membranes,  the  meninges,  with  more  or  less 
included  fluid,  envelop  the  cord.  The  spinal  nerves 
spring  from  it  in  pairs;  thirty-one  nerves  on  each  side. 
They  go  out  through  notches  in  the  bones,  one  pair 
between  each  two  vertebrae.  The  cord  is  not  so  long 
as  the  canal  in  which  it  is  lodged;  the  result  is  that  the 
nerves  which  are  to  leave  the  lower  end^of  the  canal 
descend  within  it  for  some  distance  below  the  extremity 
of  the  cord. 

A  single  spinal  nerve  is  made  by  the  union  of  two 
divisions,  or  roots,  which-*  spring  .separately  from  the 
surfaces  of  the  cord,  uniting  .as  they  leave  the  confines 
of  the  vertebrae.  These  roots  are  designated  as  dorsal 
and  ventral.  The  motor  fibers  which  it  is  our  present 
interest  to  trace  emerge  from  the  cord  in  the  ventral 
roots.  We  may,  anticipate  a  later  discussion  to  the 
extent  of  saying  that  the  dorsal  roots  are  composed 
almost  wholly  of  fibers  whose  service  is  to  carry  im- 
pulses into  the  central  axis.  There  are  thus  two  great 
classes  of  nerve  fibers,  those  which  bear  impulses  out- 
ward, efferent  fibers,  and  those  which  convey  impulses 


86 


HUMAN    PHYSIOLOGY 


FIG.  18. — At  the  left  is  shown  the  shape  of  the  spinal  cord  as  seen 
from  behind.  Two  enlargements  are  noticeable;  the  upper  is  the  re- 
gion from  which  the  arms  are  supplied,  the  lower  stands  in  relation  with 
the  legs.  Two  vertebrae  are  sketched  near  the  middle  of  the  cord  to 
show  how  it  is  enclosed  by  their  arches. 

At  the  right  is  a  cross-section  of  the  cord  with  its  dorsal  side  upmost 
and  the  H  of  gray  matter  represented,  (d.c.),  (I.e.},  and  (v.c.)  are  the 
dorsal,  lateral,  and  ventral  columns  of  the  white  matter. 


SKELETAL   MUSCLE   AND   THE   NERVOUS   SYSTEM       87 

inward,  afferent  fibers.  Not  all  the  efferent  fibers 
emerging  from  the  cord  in  the  ventral  roots  are  of 
the  motor  type  which  we  are  just  now  describing,  but 
a  large  proportion  of  them  are  of  this  sort. 

White  and  Gray  Matter. — When  a  cross-section  of 
the  spinal  cord  is  examined  with  the  naked  eye  two 
kinds  of  tissue  are  apparent  in  it.  These  could  be 
recognized  long  before  the  workers  with  the  modern 
microscope  had  resolved  them  into  their  elements. 
They  have  been  known  respectively  as  the  white  and 
the  gray  matter.  The  two  exist  not  only  in  the  cord 
but  in  the  brain.  In  the  cord  the  gray  matter  is  found 
in  all  sections  in  a  form  which  has  usually  been  likened 
to  the  letter  H.  A  diagram  will  show  how,  by  its 
presence,  it  breaks  the  white  matter  into  three  col- 
umns in  either  half  of  the  cord,  the  dorsal,  lateral,  and 
ventral  (Fig.  18). 

The  white  matter  of  the  cord  and  brain  is  nearly 
identical  with  the  substance  of  the  nerves.  That  is  to 
say,  it  is  composed  of  conducting  fibers,  closely  packed 
in  parallel  bundles.  If  it  were  not  for  the  gray  matter 
in  the  spinal  cord  we  should  be  justified  in  calling  that 
structure  the  greatest  of  all  the  nerves.  In  fact  we 
may  call  it  so  but  we  must  add  that  it  is  more  than  this. 
To  find  out  why  we  must  examine  the  gray  matter. 

The  structure  of  the  gray  matter  is  peculiarly  intri- 
cate and  obscure.  There  is  much  disagreement  with 
reference  to  its  finer  features.  Perhaps  the  first  thing 
which  commands  attention  when  it  is  viewed  under  the 
microscope  and  compared  with  the  white  matter  is  an 
apparent  looseness  of  texture.  There  are  nerve  fibers 
to  be  seen,  but  they  are  rather  widely  separated  and 
do  not  have  a  common  direction.  The  most  con- 
spicuous bodies  visible  are  the  so-called  nerve  cells. 
These  are  of  several  types.  As  a  rule  the  following 
characters  may  be  recognized.  The  outline  is  singu- 
larly broken,  each  cell  having  a  number  of  processes. 
Most  of  these  can  be  shown  to  subdivide  into  extremely 


88  HUMAN    PHYSIOLOGY 

slender  twigs,   the  dendrites.     The  nucleus  in  a  nerve 
cell  is  usually  large  and  prominent. 

.The  axons  of  nerve  fibers  are  outgrowths  of  nerve 
cells.  This  is  a  most  important  truth  to  grasp  for  it 
explains  the  relation  between  gray  and  white  matter. 
While  the  great  majority  of  the  processes  which  spring 
from  nerve  cells  branch  out  to  form  dendrites,  as  just 
described,  certain  other  processes  acquire  the  sheaths 
that  appertain  to  nerve  fibers  and  run  on  without  in- 
terruption for  long  distances.  We  can  return  now  to 


FIG.   19. — A  common  type  of  nerve  cell  giving  rise  to  the  axon  of  a 
nerve  fiber. 

the  motor  fibers  which  we  are  seeking  to  trace  to  their 
place  of  origin.  The  statement  can  now  be  made 
that  each  of  these  fibers — so  far  as  its  essential  core  or 
axon  is  concerned — is  an  outgrowth  from  a  cell  in  the 
gray  matter  of  the  cord  or  the  brain. 

Functions  of  the  Nerve  Cells. — What,  then,  are  the 
functions  which  we  are  to  ascribe  to  the  nerve  cells? 
The  impulses  which  run  along  the  fibers  must  have  come 
from  these  cells.  The  question  remains,  however, 
whether  the  cells  have  generated  the  impulses  or  merely 
transmitted  them.  We  shall  do  well  to  emphasize  the 


SKELETAL  MUSCLE  AND  THE  NERVOUS  SYSTEM       89 

conception  that  they  act  as  transmitters.  But  it  is 
possible  that  they  reinforce  the  energy  which  they  send 
forward.  One  may  very  crudely  liken  a  muscle  to  a 
torpedo  or  mine  and  its  nerve-supply  to  the  electric 
wires  provided  for  ignition.  The  nerve  cells  then  figure 
as  batteries  to  furnish  the  current.  Batteries  are 
gradually  exhausted  in  the  production  of  currents  while 
wires  are  hardly  affected  by  carrying  them.  It  has 
been  said  that  nerve  fibers  scarcely  give  evidence  of 
fatigue.  Nerve  cells  are  believed  to  suffer  some  im- 
pairment when  long  in  use. 

If  impulses  proceed  from  nerve  cells  along  motor 
fibers  to  the  end-plates  in  skeletal  muscle  and,  upon 
their  arrival,  start  the  contractile  process  we  may  be 
led  next  to  ask:  What  significance  have  the  dendrites? 
It  is  held  that  they  are  receptive  in  their  nature,  that 
through  them  the  nerve  cells  are  wrought  upon  by  stimuli. 
The  ordinary  motor  cell  has  many  dendrites  and  a 
single  axon — many  avenues  of  approach  and  only  one 
channel  of  expression.  One  is  reminded  of  the  teaching 
familiar  to  childhood  that  we  have  each  two  eyes,  two 
ears,  and  only  one  mouth.  A  nerve  cell  with  its  axon 
and  dendrites  constitutes  a  neuron. 

Nutritive  Functions  of  Nerve  Cells. — Whether  or 
not  the  nerve  cells  reinforce  the  impulses  which  they 
transmit  they  have  another  function  which  we  shall  do 
well  to  emphasize  at  this  time.  They  are  responsible 
for  the  maintenance  in  normal  condition  of  the  axons 
which  spring  from  them  and  this  is  true  no  matter  how 
long  these  axons  may  be.  It  is  a  general  truth  that 
parts  of  cells  separated  from  the  nucleated  portion  do 
not  long  survive.  If  we  regard  a  neuron  as  a  cell,  the 
impossibility  of  preserving  the  axon  when  it  is  cut  off 
from  the  cell-body  appears  merely  to  be  a  special  case 
of  this  dependence. 

When  a  nerve  is  severed  a  degeneration  of  the  fibers 
follows  and  it  is  found  to  accord  with  the  following  rule. 
Those  portions  of  the  fibers  left  in  connection  with  their 


90  HUMAN    PHYSIOLOGY 

nerve  cells  are  kept  normal;  degeneration  occurs  in  the 
parts  of  the  fibers  which  have  been  deprived  of  this 
connection.  In  most  nerves  all  the  related  cells  are  in 
the  central  axis  or  in  ganglia — detached  nodules  of  gray 
matter — near  it;  degeneration  will  therefore  be  periph- 
eral to  the  cut  and  will  involve  all  the  fibers  in  that 
direction.  The  principle  of  degeneration  has  had  a 
value  for  students  of  the  nervous  system.  It  has  been 
practicable  to  make  carefully  defined  cuts  in  the  brain 
or  the  cord  and  by  observing  the  changes  occurring  in 
the  course  of  some  weeks  afterward  to  show  that  in 
certain  bundles  the  governing  cells  have  been  left  above 
and  in  others  below  the  injury.  The  animal  must  be 
kept  alive  until  the  process  is  completed  and  then  killed 
for  postmortem  study. 

Regeneration. — A  nerve  which  has  been  cut  and  which 
has  lost  its  normal  character  throughout  the  course 
from  the  incision  to  the  endings  may  grow  again.  In 
its  degenerate  condition  it  is  still  represented  by  a  strand 
of  modified  tissue.  If  there  is  no  distinct  obstacle  to  the 
new  development  an  extension  of  the  cut  fibers  may  be 
effected  along  their  original  track.  It  is  certainly  re- 
markable that  several  thousand  fibers  may  be  so  guided 
within  their  connective-tissue  sheath  as  to  make  dis- 
tant and  useful  connections.  Within  the  central  nervous 
system  degenerated  fibers  are  not  renewed.  This  sets 
a  limit  to  the  process  of  recovery  from  injuries  to  the 
brain  and  cord.  Yet  the  outlook  is  often  better  than 
might  be  assumed  for  supplementary  paths  are  brought 
into  use,  sometimes  with  surprising  success. 

Fatigue. — How  the  dendrites  of  motor  cells  are 
brought  under  stimulation  will  be  the  subject  of  the 
next  chapter.  At  this  point  we  shall  extend  somewhat 
the  ideas  of  fatigue  previously  developed.  It  has  been 
stated  that  only  in  laboratory  experiments  can  we  ob- 
serve the  behavior  of  a  muscle  apart  from  its  nervous 
connections.  Muscle  fatigue,  pure  and  simple,  is 
unknown  to  our  individual  experience.  There  is  reason 


SKELETAL  MUSCLE  AND  THE   NERVOUS  SYSTEM 


91 


to  believe  that  the  end-plates  are  vulnerable  parts  of 
the  association  and  may  set  a  limit  to  our  voluntary 
performances.  A  common  experiment  makes  this  ap- 
pear probable.  Suppose  that  a  frog's  muscle  is  pre- 
pared with  its  nerve.  We  can  then  choose  whether  we 
will  apply  our  electric  shocks  directly  to  the  muscle 
substance  or  to  the  nerve  which  will  carry  to  the  muscle 
the  effects  of  our  stimulation. 

If  we  stimulate  the  nerve  a  great  many  times  we  shall 
witness  afc  length  a  total  failure  of  response.  If  we  then 
shift  our  application  to  the  muscle  itself  we  may  find 
it  capable  of  still  further  work.  Something  has  evidently 


FIG.  20. — A  " neuromuscular  unit"  as  defined  in  the  text.  The 
motor  nerve  cell  is  united  through  its  branched  fiber  and  end-plates 
with  five  muscle  fibers.  The  typic  number  would  be  much  larger. 

fatigued  more  rapidly  than  the  muscle  proper.  Other 
experiments,  which  we  cannot  outline  here,  forbid  us 
to  think  that  the  nerve  fibers  can  have  been  injured 
and  only  one  conclusion  remains  possible:  that  the 
trouble  is  with  that  which  is  neither  muscle  or  nerve, 
the  end-plate  that  intervenes  between  the  two.  It  is 
quite  likely  that  the  comparatively  early  failure  of  the 
end-plates  protects  the  muscles  against  excessive  strain 
and  damage.  A  muscle  which  one  regards  as  tired  is 
probably  a  muscle  containing  end-plates  that  are  more 
or  less  fatigued.  There  are  still  other  elements  in  the 
fatigue  of  daily  life  which  we  shall  have  to  indicate  at 
another  time. 


92 


HUMAN    PHYSIOLOGY 


Summary. — A  skeletal  muscle  consists  of  a  vast 
number  of  working  units,  muscle  fibers,  which  are 
modified  cells.  The  activity  of 
these  fibers  is  not  spontaneous  or 
automatic  but  dictated  through  the 
nervous  system.  Each  perfect  fiber 
in  the  muscle  has  a  motor  end-plate 
which  is  the  place  where  the  stimu- 
lus is  applied.  Each  end-plate  is 
the  terminus  of  a  branch  of  a  motor 
nerve  fiber.  A  motor  nerve  fiber, 
or  more  precisely  its  axon,  is  an 
outgrowth  from  a  cell  in  the  gray 
matter  of  the  nervous  system.  Such 
a  nerve  cell,  upon  receiving  stimu- 
lation by  way  of  its  dendrites,  sends 
out  over  its  axon  the  rapidly  tra- 
velling form  of  energy  which  we 
call  the  nerve-impulse. 

A  system  composed  of  one  motor 
nerve  cell,  its  far-reaching  fiber 
which  branches  within  the  muscle, 
and  the  cluster  of  dependent  mus- 
cle fibers  with  their  end-plates  may 
be  termed  a  neuromuscular  unit. 
When  this  little  system  is  at  work, 
the  chief  expenditure  of  fuel  sub- 
stance is  in  the  muscle.  It  follows 
that  muscular  fatigue,  in  the  strict 
sense,  is  a  possibility.  But  ac- 
tivity is  accompanied  by  some  de- 
structive processes  at  the  end-plates 
and  in  the  nerve  cells.  Hence,  fa- 
tigue at  these  points  has  also  to  be 
considered.  It  is  probable  that  fa- 
tigue becomes  effective  in  the  following  order:  first,  at 
the  end-plates,  second,  in  the  muscle  substance  itself, 
third,  in  the  gray  matter.  There  is  no  doubt  that  the 


FIG.  21. — The  princi- 
ple of  reciprocal  innerva- 
tion.  The  right  eye  is 
shown;  (n)  is  the  nose. 
(C.C.)  is  the  symbol  of  a 
coordinating  center 
which  presides  over  two 
subordinate  centers. 
When  the  eye  is  to  be 
turned  out  one  of  these 
— (C.e.)  which  commands 
the  external  rectus  mus- 
cle— is  excited  (+);  the 
center  for  the  opponent 
(C.i.)  is  inhibited  (-) 
and  the  tone  of  the  inter- 
nal rectus  is  abated. 
(Page  93) 


SKELETAL   MUSCLE   AND   THE    NERVOUS   SYSTEM 


93 


nerve  fibers,  as  found  in  the  white  matter  of  the  cen- 
tral axis  and  in  the  nerves,  have  the  greatest  resistance 
to  injury  by  use. 

Reciprocal  Innervation. — It  will  be  recalled  that  the 
skeletal  muscles  are  generally  organized  into  opposing 
groups.  It  has  also  been  said  that  these  muscles  are 
rarely  so  completely  relaxed  as  they  can  be.  In  other 
words,  they  have  tone.  Now  it  is  an  interesting  fact 
that  when  a  movement  is  made — perhaps  the  bending 
of  the  elbow — just  as  the  muscles  directly  responsible 
for  the  movement  go  into  contraction  there  is  an  aboli- 
tion of  tone  in  the  antagonistic  muscles.  Thus  they  are 
kept  from  hindering  the  act.  This  is  an  instance  of 
inhibition;  it  is  supposed  that  the  nerve  cells  presiding 
over  the  muscles  which  show  the  extra  relaxation  are 
restrained  at  this  moment  from  a  habitual,  mild  ac- 
tivity which  expresses  itself  in  the  ordinary  muscular 
tone.  "The  Law  of  Reciprocal  Innervation"  is  to  the 
effect  that  when  any  muscles  are  thrown  into  contrac- 
tion through  the  agency  of  the  central  nervous  system 
their  opponents  are  inhibited. 

This  law  has  been  verified  for  the  slender  muscles 
which  rotate  the  eyeball.  They  are  six  in  number. 
One  is  so  placed  as  to  turn  the  eye  toward  the  temple 
while  its  antagonist  can  turn  it  toward  the  nose.  The 
contraction  of  either  one  of  these  muscles  can  be  shown 
to  be  simultaneous  with  a  slackening  of  its  mate.  Of 
course  this  is  true  only  when  the  action  is  controlled 
through  the  gray  matter;  there  would  be  no  trace  of  it 
if  local  electric  stimulation  were  employed. 


CHAPTER  VII 
REFLEXES 

_In  the  previous  chapter  we  had  chiefly  in  view  the 
part  of  the  nervous  system  which  directly  controls  the 
skeletal  muscles.  The  statement  was  made  that  the 
fibers  which  establish  this  connection  are  called  efferent 
(Latin  efferre,  to  bear  away).  There  are  fibers  in  even 
greater  number  which  provide  for  the  conveyance  of 
impulses  into  the  central  nervous  system  from  localities 
external  to  it.  These,  as  has  been  said,  are  called  afferent 
afferre,  to  bear  toward).  They  are  often  called  sensory. 
An  objection  may  be  raised  against  the  term  sensory 
inasmuch  as  it  suggests  that  there  is  conscious  recogni- 
tion of  the  arrival  of  all  the  impulses  that  traverse  these 
fibers.  This  cannot  possibly  be  maintained. 

Receptors. — Afferent  fibers  are  said  to  lead  from 
receptors  to  the  brain  or  the  cord.  A  receptor  is  any 
mechanism  by  which  external  forces  can  give  rise  to 
nerve-impulses.  The  name  may  be  applied  to  the 
simple  ending  of  a  nerve  fiber  that  lies  exposed  to  pres- 
sure in  the  skin  or  it  may  refer  to  such  elaborate  organs 
as  the  eye  and  the  ear.  The  word  external  which  has 
been  used  must  be  applied  with  regard  to  the  central 
nervous  system.  The  position  of  a  receptor  may  be 
internal  as  judged  by  ordinary  standards.  Thus  there 
are  nerve-terminals  in  the  peritoneum  which  may  be 
stimulated  by  tension  and  give  rise  to  pain.  Move- 
ments of  the  head  may  affect  receptors  in  the  depths 
of  the  temporal  bone.  Yet  it  remains  true  that  we 
are  usually  concerned  with  receptors  at  the  surface  of 
the  body.  The  word  sense-organ  may  be  used  as  a 
synonym  for  receptor  though  it  is  likely  to  suggest  more 
readily  the  highly  specialized  features  of  the  equipment. 

94 


REFLEXES- 


95 


The  beginner  naturally  thinks  of  the  sense-organs  as 
existing  for  the  acquisition  of  knowledge.  This  is 
certainly  an  important  aspect  ofttheir  employment  and 
one  which  we  cannot  afford  to  slight  in  our  later  treat- 
ment of  the  subject,  but  we  must  recognize  first  a  lowlier 
type  of  action.  This  is  the  reflex.  We  must  take 
great  pains  to  define  and  illustrate  what  is  meant. 


FIG.  22. — The  principle  of  reflex  action.  The  subject  touches  a  hot 
object  (H).  Afferent  nerve-impulses  travel  the  route  marked  by  dots 
and  dashes  to  the  spinal  cord  (»S).  Efferent  impulses  return  promptly 
along  the  route  marked  by  little  crosses  to  the  muscle  (M),  which 
cooperates  with  others  not  shown  to  withdraw  the  finger  from  the 
stimulating  surface.  The  situation  of  the  coordinating  center  is  left 
undetermined,  whether  in  the  brain  or  the  cord. 

Examples   are   numerous  enough   but  they  are   apt  to 
admit  of  misconceptions. 

A  reflex  act  is  one  which  is  executed  in  response  to  an 
external  stimulus.  It  is  desirable  to  add  that  it  is  an 
act  not  requiring  attention.  Take  for  instance  the  case 
most  often  cited,  that  of  the  quick  withdrawing  of  the 
finger  from  a  hot  object.  A  child  who  made  the  trial 
would  probably  say  that  he  felt  pain  and  took  his  finger 


96  HUMAN    PHYSIOLOGY 

away  accordingly.  But  there  is  every  reason  to  think 
that  this  is  a  false  interpretation  of  what  happened; 
the  movement  occurred  and  the  pain  was  felt  after- 
ward. There  was  no  reasoning  process,  no  entertain- 
ment of  a  wish,  before  the  finger  was  pulled  away. 
Turn  to  the  case  of  the  narrowing  of  the  pupil  when 
one  raises  the  eyes  toward  the  window.  Here  is  an 
act  which  cannot  be  performed  at  will  and  one  which 
may  not  be  attended  by  any  well-marked  sensation. 
It  is  one  of  the  best  illustrations  of  the*  reflex. 

We  are  attempting  to  enforce  the  teaching  that  re- 
'  flexes  do  not  depend  on  will  or  attention.  Another  way 
of  saying  the  same  thing  is  that  they  are  the  result 
of  structure  rather  than  intelligence.  A  child  might 
think  that  a  mouse  was  seized  by  a  trap  because  the 
trap  desired  to  make  the  capture.  An  older  person 
knows  that  the  trap  necessarily  snaps  upon  its* victim 
when  a  certain  spring  is  pressed.  Its  structure  and 
not  its  will  is  responsible  for  what  takes  place.  To  an 
extent  that  is  rarely  appreciated  this  is  true  of  the 
nervous  system.  It  is  because  its  reactions  are  so  timely 
and  serviceable  that  we  find  it  difficult  to  look  upon 
them  as  mechanical. 

We  must  now  consider  the  simplest  combination  of 
elements  that  can  account  for  a  reflex.  There  must  be 
first  of  all  a  receptor  on  which  the  causative  stimulus 
can  be  brought  to  bear.  This  may  be  nothing  more 
than  the  exposed  ending  of  an  afferent  fiber,  as  has 
been  said.  There  must  be,  second,  a  path  to  the  central 
nervous  system.  Theoretically  a  single  afferent  fiber 
will  suffice.  If  this  fiber  is  one  which  enters  the  cord 
it  will  be  found  to  pass  in  by  one  of  the  dorsal  roots. 
Each  of  these  roots  has,  just  where  it  parts  company 
with  the  ventral  root,  an  elargernent  or  ganglion.  Study 
with  the  microscope  has  "shown*  that  the  fibers  of  the 
dorsal  root  run  unbroken  through  the  ganglion  but  each 
one  in  passing  connects  by  a  side  branch  with  a  nerve 
cell  within  its  compass.  These  cells  have  no  dendrites. 


REFLEXES 


97 


The  fiber  which  we  have  chosen  as  a  type  of  its  order 
continues  from  the  ganglion  to  the  dorsal  portion  of 
the  cord.  Within  this  part  of  the  nervous  axis  it  is 
said  to  branch  into  an  ascending  and  a  descending  divi- 
sion. These  run  in  the  dorsal  columns  of  the  white 


FIG.  23. — The  upper  figure  suggests  the  simplest  possible  basis  for 
reflex  action.  An  afferent  fiber,  entering  through  a  dorsal  root,  comes 
into  relation  with  the  cell-body  l^dng  rise  to  a  motor  fiber.  This  leaves 
the  cord  by  way  of  the  ventral  root. 

The  lower  figure  shows  the  branching  form  of  the  afferent  fiber  and 
the  possibility  that  it  may  play  upon  a  number  of  motor  cells  at  differ- 
ent levels. 


matter.  Here  we  have  evident  provision  for  the  in- 
troduction of  impulses  into  the  cord  and  we  know  the 
cells  of  the  neuromuscular  apparatus  are  close  by. 
It  remains  to  show  how  a  connection  can  be  established. 
It  has  been  shown  by  the  histologists  that  the  af- 
ferent fibers  in  the  cord  send  branches  at  intervals  into 


98  HUMAN    PHYSIOLOGY 

the  gray  matter.  The  manner  of  their  ending  is  some- 
what in  doubt  but  in  all  probability  they  lead  to 
the  dendrites  of  various  cells.  Impulses  led  into  the 
nervous  system  by  afferent  fibers  can  thus  be  carried 
to  the  receiving  processes  of  those  nerve  cells  which 
preside  over  the  muscles.  The  linked  system  of  afferent 
and  efferent  units  is  said  to  constitute  a  reflex  arc. 
Some  investigators  think  that  the  dendrites  are  con- 
tinuous with  a  network  everywhere  present  in  the 
gray  matter  and  that  the  fine  terminal  branches  of 
the  afferent  fibers  also  run  into  this  network.  Such  a 
view  permits  the  belief  that  an  impulse  may  wind  its 
way  through  the  gray  matter  between  its  place  of  en- 
trance and  the  place  of  its  ultimate  escape  without  any 
interruption. 

It  has  been  held  more  generally  that  the  impulses 
which  come  out  to  call  the  muscles  into  play  are  not 
the  same  impulses  that  went  in  a  moment  before.  They 
are  supposed  to  have  had  their  origin  in  particular  acts 
or  " discharges"  of  the  motor  cells  under  the  influence 
of  stimuli  applied  to  the  dendrites.  Whichever  concep- 
tion finally  prevails  we  must  believe  that  there  are  paths 
of  easy  transmission  between  certain  afferent  and  certain 
efferent  channels.  It  is  these  paths  which  make  the 
reflexes  under  most  conditions  so  advantageous. 

Synapses. — The  term  synapse  is  used  to  denote  the 
junction  of  two  units  in  the  nervous  system.  A  synapse 
is  said  to  exist  where  the  fine  terminations  of  an  efferent 
fiber  are  joined  to  the  dendrifes  of  a  motor  cell.  It  is 
not  certain  how  far  this  meeting  of  the  two  structures 
is  a  literal  joining  but  it  is  a  junction  from  the  stand- 
point of  functional  capacity.  The  nervous  system  com- 
prises an  innumerable  host  of  elements  each  influenced 
by  a  number  of  others  and  influencing  still  different 
ones  in  its  turn.  We  say  that  the  communication  is 
through  synapses  and  we  picture  them  as  in  the  dia- 
grams that  accompany  this  chapter.  We  have  at  least 


REFLEXES  99 

the  same  right  to  do  this  that  the  organic  chemist  has 
to  his  molecular  formulae. 

There  are  two  or  three  properties  which  we  associate 
with  synapses  and  we  cannot  reason  clearly  about  the 
working  of  the  nervous  system  excepting  as  we  keep 
our  faith  in  these  characters.  First  of  all,  we  believe 
that  synapses  transmit  effects  in  only  one  direction. 
They  have  been  described  as  having  a  "  valve  action." 
A  nerve  cell,  therefore,  cannot  affect  the  cells  which 
affect  it  but  always  plays  upon  others  and  these  in  turn 
must  send  impulses  over  new  courses.  The  student 
is  likely  to  think  that  it  is  the  nerve  fibers  which  have 
this  significant  polarity  but  it  is  not;  the  fibers  can  con- 
duct in  both  directions.  It  is  the  synapses  which  refuse 
to  reverse  the  transmission. 

Another  property  of  the  synapse  is  that  of  variable 
resistance.  It  is  clear  that  the  nervous  system  reacts 
much. more  readily  at  some  times  than  at  others.  Stimu- 
lants like  coffee  and  strychnin  make  it  easier  to  evoke 
reactions.  Depressants  like  alcohol  in  considerable 
doses,  or  the  anesthetics,  suppress  the  reflexes  in  a 
definite  order.  We  say  that  the  stimulants  lower  and 
the  narcotics  raise  the  central  resistance  and  we  have 
reason  to  think  that  the  changes  are  most  marked  at 
the  synapses.  If  the  adjustments  are  to  be  made  for 
the  best  interests  of  the  organism  the  average  resistance 
of  the  nervous  system  must  be  neither  too  high  nor 
too  low.  If  it  is  too  high  the  stimuli  will  fail  of  due  effect, 
while  if  it  is  too  low  the  responses  will  be  exaggerated 
and  disorderly. 

A  person  who  is  sensitive  to  coffee  may  "jump"  at 
the  slamming  of  a  door  when  he  is  under  the  influence 
of  the  stimulant.  This  is  an  unprofitable  reaction. 
A  man  poisoned  with  strychnin  may  be  thrown  into 
exhausting  convulsions  by  very  slight  causes.  These 
are  examples  of  reflex  action  but  they  no  longer  show 
it  as  ministering  to  the  welfare  of  the  body.  At  the 
other  extreme  we  have  the  grossly  intoxicated  man  who 


100  HUMAN    PHYSIOLOGY 

cannot  be  roused  to  meet  an  emergency.  Central  re- 
sistance must  be  of  a  medium  grade  if  all  is  to  go  well 
with  the  individual. 

A  subject  who  is  described  as  " nervous"  is  one  whose 
resistance  is  more  or  less  lowered  from  the  normal.  He 
will  be  unreasonably  disturbed  by  external  conditions 
which  a  more  robust  person  readily  ignores  and  he  will 
tire  himself  by  countless  reactions  which  were  better 
not  carried  out.  Low  resistance  is  seen  to  be  opposed 
to  conservation  of  one's  resources.  It  is  undoubtedly 
more  common  as  a  constitutional  fault  than  a  high 
resistance  which  must  be  indicated  by  stolidity  of 
temperament. 

Resistance  and  Coordination. — Reflex  acts  are,  in 
most  cases,  clearly  coordinated.  That  is  to  say,  a 
number  of  muscles  are  used  in  their  execution  and  their 
effects  are  combined  for  a  common  end.  Some  muscles 
bear  the  brunt  of  the  duty  while  others  support  them 
inconspicuously.  Certain  ones  act  in  advance  of  others, 
for  coordination  is  a  matter  of  sequence  as  well  as  of 
combination.  The  facts  observed  can  be  explained 
upon  the  basis  of  "  graded  synaptic  resistance."  When 
a  stream  of  afferent  nerve-impulses  enters  the  central 
gray  matter  many  possible  ways  lie  open  to  their  further 
flow.  But  some  of  these  paths  are  relatively  easy  and 
others  difficult.  The  impulses  which  find  a  free  pas- 
sage dictate  the  more  powerful  muscular  responses; 
those  which  encounter  a  greater  hindrance  find  a  more 
limited  expression  on  the  efferent  side. 

It  has  been  shown  that  the  convulsions  of  strychnin 
poisoning,  already  referred  to,  are  the  result  of  a  re- 
duction of  resistance  in  the  central  stations  such  that 
the  paths  which  are  ordinarily  impossible  to  penetrate 
are  freely  opened  while  those  that  have  normally  a 
high  resistance  come  to  have  no  more  than  the  beaten 
tracks.  We  should  anticipate  just  such  a  result  as  that 
which  we  actually  see,  opposing  muscles  straining  use- 
lessly, one  against  another. 


EEFLEXES  101 

Resistance  and  Habit. — A  reflex  act  is  one  which  is 
determined  by  circumstances  and  by  the  organization 
of  the  nervous  system.  The  same  can  be  said  of  a  habit. 
A  strong  light  keeps  the  pupil  contracted  and  a  dish 
of  candy  at  the  elbow  causes  one  to  keep  taking  pieces 
from  it.  The  principle  is  similar.  But  we  can  usually 
distinguish  without  confusion  between  a  reflex  and  a 
habit.  We  keep  the  first  term  for  those  reactions  which 
are  inborn  or  acquired  very  early  in  life  by  all  normal 
individuals.  Habits  are  established  later  and  are  more 
variable.  They  are  personal  while  reflexes  are  racial. 
It  is  plain,  however,  that  the  fundamental  condition 
for  a  habit  is  a  path  of  low  resistance  which  favors  a 
certain  action  when  definite  stimuli  are  operative. 

The  reflex  principle  is  recognizable  in  nearly  all  our 
behavior.  We  do  not  like  to  have  it  so  for  it  seems  to 
make  us  creatures  of  circumstance  rather  than  masters 
of  our  situation.  Yet  we  must  admit  that  external 
forces  guide  us  in  the  performance  of  many  acts  which 
we  confidently  call  voluntary.  When  we  walk,  our 
muscular  contractions  are  modified  every  moment  by 
stimuli  from  many  sources.  Some  of  these  we  shall 
have  to  analyze  a  little  later.  A  child  who  is  painfully 
copying  a  word  from  the  blackboard  is  really  making 
movements  which  are  shaped  by  the  visual  stimuli  he 
receives.  But  we  feel  instinctively  that  it  is  a  long  re- 
move from  the  simple  reflex  to  an  instance  like  this.  The 
question  that  cannot  be  put  aside  is:  What  is  our  will 
and  how  far  is  it  a  positive  force  in  shaping  our  conduct? 
The  physiologist  defers  to  the  philosopher  at  this  point, 
but  as  a  human  being  he  knows  that  one's  conscience  and 
one's  fellow  men  applaud  a  faith  in  one's  moral  freedom. 

If  an  act  is  not  to  be  classed  as  reflex,  in  any  degree 
whatever,  it  must  be  one  in  which  the  origin  is  clearly 
central  and  unaided  by  any  afferent  impulses.  We 
are  disposed  to  think  that  this  condition  is  realized  in 
those  actions  which  we  call  voluntary;  yet  from  another 
point  of  view  these  can  be  regarded  as  delayed  reflexes. 


102  HUMAN    PHYSIOLOGY 

They  are  determined  by  external  stimuli  some  of  which 
may  have  been  brought  to  bear  a  long  time  before  the 
occasion.  A  man  somewhat  suddenly  decides  to  walk 
up  a  hill  to  enjoy  a  prospect  which  he  has  seen  the 
previous  year.  We  describe  the  act  as  voluntary  but 
at  least  one  of  the  factors  concerned  in  causing  it  is 
the  impression  made  upon  his  nervous  system  at  the 
time  of  his  earlier  visit.  So  the  discussion  of  reflexes 
leads  not  only  to  the  problems  of  habit  but  to  those  of 
memory.  This  subject  will  be  more  advantageously 
considered  in  connection  with  the  cerebrum. 

Examples  of  Reflex  Action. — We  have  spent  a  good 
deal  of  time  in  treating  the  topic  in  general  terms. 
Let  us  now  turn  to  some  specific  illustrations.  What 
are  some  of  the  reflexes  exhibited  by  a  baby  and  how 
do  they  make  for  its  welfare?  One  thinks  imme- 
diately of  the  sucking  reflex.  The  infant  sucks  vigor- 
ously whatever  is  put  between  its  lips.  In  the  natural 
course  of  events  this  reaction  insures  a  supply  of  food. 
As  it  eats  it  frequently  chokes  and  coughs;  the  cough  is 
a  reflex  which  prevents  the  entrance  of  foreign  materials 
into  the  breathing  passages.  A  similar  reflex — sneez- 
ing— is  calculated  to  expel  obstructing  substances  from 
the  nose.  Vomiting  is  a  reflex  which  relieves  the  over- 
filled stomach.  Crying  has  a  less  obvious  function 
unless  we  assume  that  it  secures  the  attention  of  the 
parents  to  needs  which  the  child  by  itself  cannot  satisfy. 

Some  of  the  reflexes  which  are  prominent  in  the  baby 
are  disguised  or  replaced  by  others  during  the  period  of 
growth.  Sucking  ceases  to  be  a  predictable  perform- 
ance. Crying  is  less  and  less  readily  induced.  The 
other  responses  which  we  have  mentioned  continue  to 
occur.  So,  too,  the  reactions  of  the  pupils  and  the 
withdrawal  of  parts  of  the  body  from  objects  that 
threaten  injury  are  retained.  In  the  course  of  a  year 
or  two  the  child  has  the  capacity  to  keep  its  feet  and  to 
walk,  attainments  based  largely  upon  the  development 
of  reflex  mechanisms. 


REFLEXES  103 

Reflexes  Other  than  Movements.— So  far  we  have 
spoken  of  reflexes  as  though  they  were  necessarily 
acts  of  the  muscles.  The  conception  must  now  be 
made  broader.  Certain  reflexes  have  a  negative  or 
inhibitory  character.  A  dash  of  cold  water  may  cause 
one  to  hold  the  breath  momentarily  and  perhaps  at  the 
same  time  the  heart  may  "  drop  a  beat."  The  suspension 
of  breathing  and  the  omission  of  the  heart's  contraction 
are  true  reflexes,  but  instead  of  being  movements  they 
consist  in  the  suppression  of  movements  which  were 
due  to  occur. 

There  are  also  reflexes  which  are  executed  by  glands 
instead  of  muscles.  A  gland  is  an  organ  which  pre- 
pares and  discharges  some  chemical  product.  Either 
the  action  of  the  gland  or  its  product  may  be  described 
by  the  word  secretion.  Some  glands,  like  the  kidneys 
and  the  liver,  continuously  evolve  their  secretion;  others 
nearly  or  quite  intermit  activity.  Glands  of  the  latter 
type  are  often  found  to  be  as  distinctly  subject  to 
nervous  government  as  are  the  skeletal  muscles.  The 
nerves  which  stretch  out  to  them  from  the  central 
organs  are  spoken  of  as  secretory  nerves.  They  are 
efferent  but  not  motor  according  to  our  definitions. 

One  of  the  most  familiar  examples  of  the  reflex  ex- 
citation of  secretion  is  afforded  by  the  flow  of  tears  when 
this  results  from  a  cinder  in  the  eye.  As  there  are 
winking  movements  at  the  same  time  we  have  here  a 
particularly  good  demonstration  of  two  kinds  of  reflex 
action.  When  the  tears  start  from  emotional  causes  the 
reflex  character  of  the  act  is  not  so  plain.  A  hot  or  an 
acid  fluid  taken  into  the  mouth  produces  a  reflex  secre- 
tion of  saliva.  Warming  the  skin  will  call  forth  a  reflex 
discharge  of  perspiration. 

Psycho-reflexes. — In  characterizing  the  standard 
reflex  we  insisted  that  consciousness  does  not  enter  into 
it  as  an  essential  feature.  We  may  notice  our  own  re- 
flexes but  to  observe  is  not  to  control  them.  Often 
they  run  counter  to  our  desires;  we  may  be  compelled 


104 


HUMAN    PHYSIOLOGY 


to  cough  or  sneeze  when  we  would  much  rather  not. 
Yet  there  are  reactions  which  we  are  inclined  to  call 
reflex  which  nevertheless  depend  much  upon  the  color- 
ing of  our  consciousness.  Take,  for  example,  the  water- 
ing of  the  mouth  at  the  sight  of  delicious  food — or 
of  a  lemon.  Here  is  a  response  which  is  determined  by 
external  stimuli  but  which  would  not  take  place  in  an 
unconscious  nor  even  in  an  inattentive  subject.  The 
secretion  of  gastric  juice  normally  accompanies  the 
taking  of  food  but  only  when  there  is  an  element  of 


FIG.  24. — This  differs  from  the  second  diagram  in  Fig.  23  in  the  fol- 
lowing respect.  The  afferent  fiber  does  not  connect  directly  with  the 
motor  cells  but  with  intermediate  or  association  neurons  whic~h  extend 
the  effect  of  the  original  impulse  to  groups  of  efferent  elements. 

pleasure.     The   term   psycho-reflex   is   applied   to    such 
adaptive  changes  as  these. 

The  Position  of  Reflex  Arcs. — Our  conventional 
diagrams  usually  represent  segments  of  the  spinal  cord 
as  containing  the  synapses  through  which  reflexes  are 
brought  about.  We  are  entirely  warranted  in  this 
representation,  for  certain  reflexes  can  be  mediated 
by  the  spinal  cord  when  it'  has  been  separated  from  the 
brain.  Still,  it  would  be  wrong  to  leave  the  impression 
that  the  cord,  in  the  higher  animals,  plays  the  leading 
part  in  coordinating  incoming  with  outgoing  impulses. 


REFLEXES  105 

The  reflex  principle  is  recognized  in  the  brain  as  well 
as  in  the  cord  and,  in  our  own  case,  reflexes  that  are 
purely  regulated  by  the  cord  are  not  usual. 

Association  Units. — A  diagram  modified  to  suggest 
the  path  followed  by  impulses  when  the  brain  figures 
in  reflex  action  should  show  more  than  two  orders  of  units. 
Thus  far  we  have  pictured  the  afferent  element  as  bring- 
ing it's  influence  to  bear  directly  upon  the  efferent.  It 
is  not  likely,  even  in  the  cord,  that  this  simple  relation- 
ship is  the  common  one.  Intermediate  nerve  cells  are 
ordinarily  concerned  in  the  transmission.  If  this  is 
so,  then,  as  a  rule,  more  than  one  order  of  synapses 
will  lie  in  the  path.  The  intermediate  cells,  which  are 
not  dearly  afferent  or  efferent,  have  been  called  ad- 
justors  or  association  units.  As  more  and  more  of  these 
are  introduced  we  have  more  tendency  toward  varia- 
tion in  reflex  action  and  we  find  that  we  are  moving 
from  a  lower  to  a  higher  type  of  nervous  system. 

Reflex  Time. — We  can  measure  with  great  accuracy 
the  time  that  elapses  between  the  giving  of  a  stimulus, 
such  as  an  electric  shock,  and  the  beginning  of  a  reflex 
movement  in  response.  The  trial  is  often  made  for 
the  wink.  The  shock  is  applied  to  the  eyelid  and  the 
twitch  that  follows  makes  a  written  record.  By  means 
of  the  tuning  fork  the  interval  can  be  estimated.  It 
is  likely  to  be  about  0.05  second.  Many  reflexes  are 
slower,  few,  if  any,  are  faster  than  this.  The  course 
run  by  the  nerve-impulses  from  the  eyelid  to  the  lower 
part  of  the  brain  and  back  to  the  muscles  is  a  short  one; 
most  of  the  time  seems  to  be  consumed  at  the  synapses 
and  in  the  nerve  cells  rather  than  in  transit. 

If  the  wink  following  an  electric  shock  is  not  in- 
voluntary but  made  "on  purpose,"  as  we  say,  the  delay 
is  something  like  three  times  as  great  as  with  the  pure 
reflex — say  0.15  second.  It  is  then  what  we  call  a 
voluntary  reaction  time.  The  impulses  must  have  gone 
over  much  longer  and  more  interrupted  routes. 


CHAPTER  VIII 
THE  BRAIN 

The  student  should  now  be  able  to  picture  to  himself 
the  following  large  features  of  the  central  nervous  sys- 
tem of  man.  The  brain  is  enclosed  within  the  skull  and 
the  spinal  cord  descends  from  it  through  a  bony  canal 
formed  by  the  arches  of  the  vertebrae.  From  the  cord 
spring  to  right  and  left  31  pairs  of  nerves.  Each  one 
is  a  cable  composed  of  thousands  of  fibers.  Somewhat 
more  than  half  these  fibers  are  arranged  to  convey 
impulses  from  receptors  to  the  cord;  the  remainder 
are  used  to  carry  impulses  out  to  muscles  and  glands. 
(Muscles  and  glands  are  sometimes  called  effectors.) 

Close  to  the  cord  each  spinal  nerve  cleaves  into  a 
dorsal  and  a  ventral  root.  The  dorsal  root  bears  a 
ganglion  containing  the  cells  with  which  the  afferent 
fibers  are  united.  The  ventral  root  is  made  up  almost 
wholly  of  efferent  fibers.  It  will  be  seen  that  a  spinal 
nerve,  considered  external  to  the  place  of  union  of  its 
two  roots,  is  mixed  in  character,  containing  both  af- 
ferent and  efferent  fibers.  When  a  nerve  branches, 
its  fibers  do  not  branch  but  are  merely  parted.  Yet 
nerve  fibers  may  branch,  as  we  have  seen,  within  the 
central  nervous  system  and  also,  in  the  case  of  the 
motor  elements,  when  they  approach  the  end-plates 
in  skeletal  muscle. 

As  there  are  spinal  nerves  springing  from  the  cord  so 
there  are  nerves  which  originate  from  the  brain.  These 
are  known  as  the  cranial  nerves.  They  emerge  through 
openings  in  the  skull  and  are  distributed  to  localities  in 
the  head  and  neck.  One  of  them,  on  each  side,  departs 
from  this  rule  since  it  runs  down  into  the  trunk.  This  is 

106 


THE   BRAIN 


107 


the  vagus,  the  tenth  in  the  series.  In  all  there  are  twelve 
pairs  of  cranial  nerves.  They  are  of  very  unequal  size 
and  do  not  show  the  regular  separation  into  roots  that 
has  been  described  for  the  spinal  system.  In  the  cranial 
nerves  there  is  an  overwhelming  preponderance  of 
afferent  fibers.  This  is  associated  with  the  existence  of 
the  important  sense-organs  of  the  head,  the  eye,  the  ear, 
and  the  nose. 

The  Human  Brain. — By  far  the  largest  part  of  the 
cranial  cavity  is  occupied,  in  man,  by  the  great  mass 
of  nervous  tissue  which  we  call  the  cerebrum.  This 


FIG.  25. — The  human  brain  from  the  left.  The  main  mass  is  the 
left  cerebral  hemisphere.  The  cerebellum,  with  its  narrow  convolu- 
tions, is  below  and  behind.  The  medulla  bears  the  stumps  of  several 
cranial  nerves. 

is  the  " highest"  division  of  the  brain  in  an  anatomic 
and  in  a  physiologic  sense.  At  the  first  glance  one  is 
impressed  by  its  curiously  convoluted  surface.  It  is 
deeply  scored  by  winding  furrows.  A  profound  cleft, 
running  longitudinally  in  the  mid-plane,  divides  it 


108  HUMAN    PHYSIOLOGY 

into  two  hemispheres,  right  and  left.  The  cerebrum 
is  united  below  with  the  spinal  cord  by  means  of  an 
intermediate  link  which  is  conveniently  referred  to 
as  the  brain-stem.  Several  subdivisions  can  be  recog- 
nized in  the  brain-stem  and  mention  may  be  made  of 
the  one  which  is  next  the  cord.  This  is  the  medulla. 
At  the  back  of  the  cerebrum  and  overhung  by  it  is 


FIG.  26. — The  brain  cut  lengthwise  in  the  mid-plane;  in  other  words, 
the  right  half  seen  from  the  left.  Again  the  cerebrum  is  the  chief 
mass;  the  arched  structure  (c.c.)  is  the  chief  bond  between  the  hemi- 
spheres. The  cerebellum  presents  a  leaf-like  pattern.  Cavities  can 
be  traced  in  the  brain-stem. 

a  rather  large  division  of  the  brain  known  as  the  cere- 
bellum. Its  surface  is  marked  by  numerous  fine  corru- 
gations. The  cerebellum  is  furnished  with  abundant 
connections  with  the  brain-stem  and  so,  less  directly, 
with  the  cerebrum  above  and  the  cord  below.  The 
general  ordering  of  the  parts  of  the  brain  is  best  ap- 
preciated from  a  diagram  of  a  median  section,  or,  better 
still,  from  a  model  which  can  be  parted  in  this  plane. 


THE    BRAIN  109 

The  Cranial  Nerves. — Of  the  cranial  nerves,  only 
one  pair  join  the  cerebrum.  These  are  the  olfactory 
nerves,  diffuse  collections  of  fibers  which  lead  into  the 
brain  from  microscopic  receptors  in  the  upper  part 
of  the  nasal  passages.  As  the  name  implies,  these  nerves 
are  responsible  for  the  sense  of  smell.  The  olfactory 
nerves  are  the  farthest  forward  of  all  in  the  cranial 
series,  so  they  are  spoken  of  as  the  first  cranial  nerves. 
The  second  pair  are  the  optic  nerves,  trunks  of  large  size 
coming  from  the  eyeballs  to  a  place  on  the  under  sur- 
face of  the  brain  where  they  appear  to  cross  in  the  form 
of  a  letter  X.  Their  fibers  enter  the  brain-stem  close 
to  the  cerebrum.  The  remaining  cranial  nerves  are 
connected  with  the  brain-stem  at  short  intervals  be- 
tween the  place  of  entrance  of  the  optic  fibers  and  the 
opening  at  the  base  of  the  skull  through  which  the  spinal 
cord  goes  out. 

We  need  not  make  a  complete  list  of  the  cranial  nerves 
and  the  regions  with  which  they  stand  in  relation  but 
certain  ones  may  be  noticed.  The  fifth  pair  are  the 
important  trigeminal  nerves,  bringing  in  sensory  fibers 
from  the  face,  the  outer  surface  of  the  eyeballs,  the  teeth, 
and  the  linings  of  the  mouth  and  nose.  The  seventh 
nerves  are  the  facial,  controlling  the  small  muscles  on 
which  the  play  of  facial  expression  depends.  The  dis- 
abling of  one  of  these  nerves,  or  rather  of  the  gray 
matter  related  to  it,  causes  a  drooping  of  one  side  of 
the  face,  a  condition  which  is  not  uncommon.  The 
eighth  pair  of  cranial  nerves  are  the  auditory,  valuable 
not  only  as  mediating  the  sense  of  hearing  but  bearing 
an  important  part  in  shaping  the  reflexes  that  serve  to 
maintain  equilibrium.  The  tenth  or  vagus  nerves, 
already  mentioned,  have  a  manifold  service,  as  we  shall 
see. 

White  and  Gray  Matter  in  the  Brain. — The  brain- 
stem  resembles  the  spinal  cord  in  having  white  matter 
at  the  surface.  But  the  H-figure  is  not  to  be  traced 
within;  the  gray  matter  occurs,  rather,  in  more  or  less 


110  HUMAN    PHYSIOLOGY 

isolated  collections.  Some  of  these  are  distinctly 
tributary  to  certain  cranial  nerves  and  are  described 
as  the  nuclei  of  these  nerves.  The  cerebrum  and  the 
cerebellum  have  gray  matter  spread  in  a  thin  layer 
over  their  surface,  a  development  called  the  cortex. 
We  have  every  reason  to  believe  that  it, is  upon  the 
organization  of  the  cortex,  especially  of  the  cerebrum, 
that  the  rank  of  an  animal  depends.  The  human 
cortex  is  thus  of  extraordinary  interest  to  us.  The 
convolutions  seem  to  be  devices  to  increase  the  extent 
of  cortex.  The  interiors  of  the  cerebrum  and  the  cere- 
bellum are  taken  up  chiefly  with  white  matter,  fibers 
sweeping  in  all  directions,  but  there  are  some'  submerged 
clumps  of  gray  matter  in  both  ihese  divisions.  A  series 
of  small,  communicating  cavities  can  be  followed  through 
the  brain-stem  and  into  the  hemispheres.  These  spaces 
contain  a  clear  fluid  identical  with  that  held  between  the 
layers  of  the  meninges. 

The  Medulla. — To  look  at  the  medulla  one  would 
think  it  merely  an  extension  of  the  spinal  cord.  But 
this  short  segment  of  the  nervous  axis  has  unique  powers. 
The  function  which  we  must  first  recognize  is  the  con- 
trol of  breathing.  We  say  that  the  medulla  contains 
the  respiratory  center.  This  was  inferred  about  a  hundred 
years  ago  when  it  was  shown  that  cutting  the  cord  below 
the  skull  permanently  stops  the  breathing  and  so,  in 
the  case  of  any  of  the  higher  animals,  causes  immediate 
death.  Cutting  through  the  brain-stem  just  above  the 
medulla  does  not  stop  the  breathing  and  is,  therefore, 
not  instantly  fatal.  Comparing  the  results  of  these 
two  experiments,  physiologists  have  been  led  to  believe 
that  there  is  in  the  medulla  an  important  station  from 
which  breathing  is  directed.  Its  position  has  been 
more  narrowly  defined  by  slicing  across  the  medulla 
repeatedly,  beginning  at  the  top,  and  noting  at  what 
level  the  progressive  destruction  has  abolished  the 
breathing  movements. 

The  muscles  used  in  breathing  are, .  with  minor  ex- 


THE    BRAIN  111 

ceptions,  supplied  with  motor  nerve  fibers  originating 
in  the  spinal  cord.  Hence,  the  cells  in  the  medulla 
must  not  be  thought  of  as  directly  connected  with  these 
muscles  but  with  the  lower  order  of  nerve  cells  in  the 
cord.  Here  for  the  first  time  we  have  an  example  of 
what  is  common  enough  in  many  other  mechanisms, 
a  higher  center  presiding  over  a  number  of  centers  of 
lower  rank,  as  a  colonel  commands  the  captains  in  his 
regiment.  It  may  be  added  that  in  the  respiratory 
system  the  "captains"  exercise  no  discretionary  power 
when  their  superior  ceases  to  hold  them  to  their  work. 

The  respiratory  center  is  much  subject  to  reflexes. 
Almost  any  sudden  shock  is  certain  to  modify  the  breath- 
ing in  some  way.  On  the  other  hand  we  are  not  to 
think  that  the  taking  of  each  breath  is  a  reflex"  act. 
It  is  probably  accomplished  without  the  essential  as- 
sistance of  afferent  impulses.  The  center  is  said  to 
be  automatic.  This  is  a  word  we  have  used  before  to 
describe  cardiac  and  smooth  muscle.  As  those  two 
contractile  tissues  tend  to  exhibit  a  rhythmic  type  of 
activity  in  the  absence  of  recognizable  stimulation, 
so  the  respiratory  center  seems  disposed  to  go  on  in 
the  performance  of  its  duty  without  external  prompting. 

When  we  say  that  automatic  tissues  are  active  in 
the  absence  of  external  stimuli  we  do  not  say  that  they 
are  not  responding  to  a  local  or  internal  excitation. 
We  must  suppose  that  they  are.  Chemical  condi- 
tions developing  moment  by  moment  doubtless  de- 
termine their  behavior.  Many  facts  bearing  upon  this 
matter  have  been  discovered  and  some  of  them  we 
shall  have  occasion  to  speak  of  in  discussing  the  heart- 
beat. As  regards  the  respiratory  center  the  most 
significant  point  is  its  response  to  variations  in  the 
carbon  dioxid  content  of  the  blood.  An  increased 
concentration  of  this  gas  in  the  blood  carried  to  the 
brain  produces  a  prompt  increase  in  the  breathing 
movements. 

A  diminution  of  the  carbon  dioxid  in  blood  below 


112  HUMAN 


PHYSIOLOGY 


the  usual  amount  may  cause  a  suspension  of  breath- 
ing. This  favors  the  view  that  carbon  dioxid  is  the 
normal  stimulant  for  the  respiratory  center.  It  will 
be  convenient  to  postpone  a  fuller  treatment  of  this 
subject  until  the  physiology  of  respiration  is  taken  up. 

Other  services  of  the  medulla  are  connected  with  the 
regulation  of  the  circulation.  It  will  be  recalled  that 
the  vagus  nerves  spring  from  the  medulla  and  these 
nerves  contain  many  fibers  which  extend  to  and  from 
the  heart.  Destruction  of  the  medulla  will  not  stop 
the  beating  of  the  heart  as  it  stops  the  breathing  but 
the  heart  will  be  deprive^rbf  a  certain  control  by  the 
operation.  The  influence  withdrawn  is  mainly  of  an 
inhibitory  sort  and  the  result  is  a  quickening  of  the 
heart  rate. 

Besides  exercising  a  regulating  function  upon  the 
heart  the  medulla  is  responsible  for  the  maintenance 
of  the  tone  of  the  blood-vessels.  This  fact  is  expressed 
by  saying  that  the  medulla  contains  the  vasomotor 
center.  To  destroy  the  region  to  which  this  name  is 
applied  is  to  cause  a  nearly  universal  slackening  of  the 
small  arteries  and  veins,  showing  that  they  were  pre- 
viously held  in  a  state  of  sustained  contraction.  Vaso- 
motor reflexes  can  best  be  studied  in  connection  with 
the  other  problems  of  the  circulation. 

The  medulla  is  the  seat  of  still  other  mechanisms, 
some  of  which  pertain  to  the  alimentary  system.  It 
has  to  do  with  the  execution  of  swallowing,  an  act 
which  is  more  complex  than  we  are  apt  to  appreciate. 
It  is  concerned  in  the  government  of  the  salivary  glands 
and  the  glands  in  the  lining  of  the  stomach.  And  we 
must  not  forget  that  the  medulla  is  also  a  part  of  the 
main  line  of  communication  between  the  higher  parts 
of  the  brain  and  the  body.  It  is  traversed  by  the  fibers 
which  we  employ  in  voluntary  acts  and  by  those  which 
make  possible  the  sensations  which  we  refer  to  the 
trunk  and  extremities.  Many  of  the  paths  of  trans- 
mission which  lie  in  the  medulla  cross  from  one  side  of 


THE   BRAIN  113 

the  mid-plane  to  the  other  while  passing  through.  This 
has  to  do  with  a  prominent  and  curious  truth — that  the 
right  side  of  the  cerebrum  sustains  relations  chiefly  with 
the  left  side  of  the  body  and  vice  versa. 

The  limited  portion  of  the  brain-stem  which  inter- 
venes between  the  medulla  and  the  cerebrum  is  of 
interest  to  us  as  containing  way-stations  on  the  paths 
from  the  eyes  and  the  ears  to  the  higher  centers.  This 
part  of  the  brain  is  also  known  to  preside  over  the 
muscles  which  rotate  the  eyeballs,  change  the  visual 
focus  for  different  distances,  and  alter  the  size  of  the 
pupils. 

The  Cerebellum  and  Equilibrium. — It  has  been  said 
that  the  cerebellum  is  a  dorsal  outgrowth  from  the 
brain-stem.  It  can  be  cut  away  without  interrupting 
the  direct  lines  by  which  impulses  pass  along  the  nervous 
axis.  When  the  brains  of  different  animal  species  are 
compared  we  find  that  no  correspondence  between  the 
size  of  the  cerebellum  and  intelligence  can  be  established. 
This  part  of  the  brain  is  particularly  large  in  birds  and 
fishes.  One  thinks  naturally  of  the  remarkable  loco- 
motor  powers  of  these  types,  the  coordinated  muscular 
system  of  the  fish  that  corresponds  in  number  of  ele- 
ments with  the  elaborate  skeleton  and  the  equipment  for 
flying  which  so  distinguishes  the  bird.  There  is  in 
fact  no  doubt  that  the  cerebellum  stands  in  some  re- 
lation to  locomotion  and  balancing. 

When  it  has  been  removed  from  a  pigeon  the  bird 
presents  a  distressing  picture.  It  seems  quite  as  sensi- 
tive as  a  normal  bird  and  overcome  by  panic.  It  can- 
not fly;  if  tossed  into  the  air  it  falls  fluttering  helplessly 
and  throws  itself  about  on  the  ground.  It  cannot  even 
keep  its  feet.  No  muscles  appear  to  be  paralyzed,  but 
there  is  a  lamentable  loss  of  the  ability  to  use  them  in 
groups  for  common  ends.  A  bird  that  has  undergone 
this  operation  will  injure  itself  if  it  is  not  restrained. 
If  it  is  carefully  kept  and  tended  it  improves  considerably, 
but  does  not  recover  its  original  poise  and  control. 


114  HUMAN    PHYSIOLOGY 

.  A  similar  experiment  .has  been  made  upon  the  dog. 
With  this  animal  also  the  first  impression  is  of  utter 
failure  to  coordinate  the  movements.  The  dog  rolls 
and  writhes  as  though  convulsed  with  pain.  Yet  it  is 
probably  not  suffering  unless  from  terror  and  bewilder- 
ment. After  a  time  there  is  a  great  degree  of  recovery 
but  there  is  a  permanent  unsteadiness,  awkwardness, 
and  quick  susceptibility  to  fatigue.  On  the  whole 
there  seems  to  be  no  doubt  that,  while  the  earlier  ex- 
perimenters assigned  the  function  of  equilibration  too 
exclusively  to  the  cerebellum,  it  does  have  to  do  with 
this  function.  This  will  be  a  good  time  to  outline  the 
reflex  adjustments  by  which  the  balance  is  preserved. 

A  statue  of  a  man  is  a  most  unstable  object.  It 
cannot  be  relied  on  to  keep  its  feet  when  there  are  forces 
tending  to  upset  it.  But  we  see  the  living  body  main- 
taining an  erect  position  in  the  midst  of  disturbing 
circumstances,  bracing  against  the  wind,  adapting  itself 
to  slopes,  and  saving  itself  from  falling  again  and  again. 
The  mind  is  ordinarily  but  little  occupied  with  these 
processes;  they  seem  to  take  care  of  themselves.  To  say 
this,  is  practically  to  say  that  the  reflex  principle  is 
involved.  We  shall  find  that  this  is  clearly  the  case. 

The  body  is  always  swaying  more  or  less.  Each 
oscillation  threatens  a  fall,  but  normally  there  is  a  timely 
check  and  reversal  of  the  movement.  If  this  is  a  re- 
flex we  must  seek  to  show  what  receptors  are  stimulated 
to  insure  the  reaction.  We  have  to  recognize  several 
orders.  The  following  are  certainly  to  be  included: 
(1)  those  in  the  soles  of  the  feet,  (2)  a  great  number  in 
the  muscles,  tendons,  and  joints,  (3)  the  eyes,  and  (4) 
the  internal  ears.  The  order  does  not  necessarily 
indicate  their  relative  importance. 

1.  When  one  sways  forward  the  pressure  on  the  soles 
of  the  feet  becomes  lessened  at  the  heels  and  increased 
at  the  toes.  If  the  body  tilts  to  the  right  the  pressure 
upon  that  foot  is  increased  and  the  weight  borne  by 
the  left  foot  is  diminished.  There  are  receptors  in  the 


THE   BRAIN  115 

soles  which  are  responsive  to  stimulation  by  pressure. 
The  afferent  currents  which  result  have  a  long  course 
to  run  to  reach  the  brain.  In  the  gray  matter  of  the 
cerebellum,  and  probably  in  other  places  too,  they 
generate  efferent  impulses  which  play  upon  the  motor 
cells  of  the  cord  and  secure  purposeful  contractions  of 
various  muscles  tending  to  restore  the  balance. 

2.  Any  swaying  of  the  body  alters  the  tension?  of 
many  muscles  and  their  associated  connective  tissues. 
The  bearing  upon  one  another  of  the  small  bones  in 
the  feet  is  subjected  to  change,  the  stresses  in  all  the 
leg  and  trunk  muscles  are  modified,  the  weight  of  the 
head  puts  special  and  varying  strain  upon  the  muscles 
of  the  neck.     We  must  never  forget  that  the  motor 
equipment  of  the  body  is  at  the  same  time  a  great  re- 
ceptor system.     No  muscle  is  contracted  without  regis- 
tering its  action  by  returning  impulses  to  the  cord  and 
brain.     When    a   muscle   is   stretched   by   an    external 
force  the  same  thing  is  true.     One  of  the  chief  services 
of  impulses  thus   generated  is  to  guide  the   adaptive 
reactions  that  make  for  equilibrium. 

3.  If  the  head  moves  the  retinal  pictures  are  shifted. 
Our  attention  is  not  usually  fixed  upon  this  matter  but 
if  we  attend  to  it  at  all  we  are  quick  to  interpret  the 
experience  as  due  to  the  swaying  of  the  body.     Whether 
we  realize  the  displacement  of  the  images  or  not  we 
may  not  doubt  that  it  is  one  of  the  sources  of  stimula- 
tion on  which  we  depend  to  steady  ourselves.     No  one, 
probably,   can   stand   quite  so  steadily  with  the   eyes 
closed  as  he  can  when  he  is  using  them.     Reliance  on 
the  eyes  is  more  absolute  when  the  other  mechanisms 
are  at  fault;  there  are  cases  of  nervous  disease  in  which 
the  victims  reel  and  fall  when  the  eyes  are  bandaged, 
though  able  to  stand  by  their  aid. 

The  giddiness  and  alarm  which  we  feel  when  at  the 
verge  of  a  precipice  can  be  explained  in  this  connection. 
It  is  not  wholly  the  result  of  imagined  danger.  We  are 
used  to  the  presence  of  objects  in  front  of  us  and  the 


116  HUMAN    PHYSIOLOGY 

apparent  movement  x>r  parallax  of  these  objects  is  a 
familiar  condition  of  our  life.  If  we  are  placed  where  the 
nearest  rocks  and  trees  are  many  yards  away,  before 
or  below  us,  the  seeming  movement  of  these  landmarks 
is  much  less  than  that  to  which  we  are  accustomed. 
We  miss  one  of  the  clues  to  our  own  success  or  non-suc- 
cess in  keeping  our  poise.  No  doubt  it  is  much  easier 
to  stand  at  the  brink  of  a  chasm  10  feet  wide  than  on 
the  edge  of  a  broad  abyss  of  the  same  depth.  The  dan- 
ger of  a  fall  is  no  greater  in  the  second  case  than  in  the 
first  but  we  are  greatly  sustained  by  the  visual  stimulus 
furnished  by  the  opposing  wall  of  the  chasm. 

4.  A  very  remarkable  mechanism  contributing  to 
equilibrium  exists 'in  the  internal  ear.  When  we  men- 
tioned the  auditory  nerve  a  short  time  ago  we  stated 
that  this  nerve  does  not  serve  solely  to  mediate  the 
sense  of  hearing.  One  of  its  two  divisions  has  this 
function;  the  other  is  valuable  because  of  the  corrective 
reflexes  it  produces  when  the  balance  is  endangered. 
This  branch  of  the  auditory  nerve  is  called  the  vestibu- 
lar.  Its  fibers  arise  in  the  winding  cavities  of  the 
temporal  bone  which  constitute  what  is  fitly  termed  the 
labyrinth.  These  cavities  are  filled  with  liquid.  When 
the  head  is  moved  as  one  sways,  or  more  violently 
when  one  stumbles,  the  fluid  is  shaken.  In  it  are 
delicate  filaments  which  appear  to  be  connected  with 
the  nerve  fibers  that  take  their  departure  from  the 
linings  of  the  labyrinth.  Thus  we  have  in  the  internal 
ear  a  receptor  system  which  is  very  sensitive  to  me- 
chanical disturbances  and  sends  to  the  brain  the  im- 
pulses which  result. 

We  are  not  usually  aware  of  the  nerve  currents  that 
arrive  in  the  central  nervous  system  from  the  labyrinth. 
When  we  do  become  cognizant  of  them  it  is  to  feel  the 
sensation  of  vertigo.  It  is  likely  that  the  internal  ear 
allies  itself  with  other  receptors  to  induce  sea-sickness 
when  the  body  is  persistently  moved  about  in  space  by 
the  rolling  and  pitching  of  the  vessel.  When  dizziness 


THE   BRAIN  117 

is  caused  by  revolving  the  body  the  disturbances  of  the 
ear  receptors  are  combined  with  a  curious  ocular  reac- 
tion called  nystagmus.  It  will  be  worth  while  to  say  a 
word  about  this  movement. 

If  a  person  is  spun  around,  as  in  an  office  chair,  the 
eyes  behave  in  a  characteristic  fashion.  Suppose  the 
rotation  is  toward  the  right.  The  eyes  fix  themselves 
upon  some  object  and  as  the  body  is  turned  away  from 
it  they  are  swung  in  the  opposite  direction  (toward  the 
left),  keeping  the  landmark  in  view  as  long  as  possible. 
When  the  eyes  can  be  carried  no  farther  they  are 
snapped  very  sharply  to  the  right  and  another  object 
is  seized  upon.  There  is  again  the  measured  sweep  to 
the  left  and  the  quick  snap  to  the  right.  The  name 
nystagmus  is  applied  to  this  alternating  movement  of 
the  eyes  in  which  the  travel  opposite  in  direction  to  the 
rotation  of  the  body  is  moderate  in  speed  while  the 
counter-movement  is  extremely  rapid. 

When  the  subject  of  such  an  experiment  is  brought 
to  rest,  the  nystagmus  continues  for  a  time.  It  is 
accompanied  by  the  distressing  illusion  that  "  every- 
thing is  going  round  and  round."  The  uncontrollable 
movements  of  the  eyes  continually  shift  the  images  upon 
the  retinas.  It  is  thought  that  the  quick,  snapping 
motions  are  not  sources  of  sensation  while  the  slower 
ones  are  vividly  effective.  The  result  is,  accordingly, 
the  impression  that  the  surroundings  are  revolving  in 
one  direction  rather  than  oscillating  back  and  forth. 
By  closing  the  eyes  one  lessens  but  does  not  entirely 
do  away  with  the  feeling.  When  the  shifting  images  can 
no  longer  be  seen  there  are  still  sensations  from  the  eye 
muscles  as  they  go  on  with  their  unprofitable  working. 

We  are  not  justified  in  saying  that  the  cerebellum  is 
the  only  central  station  through  which  the  reflexes  mak- 
ing for  equilibrium  are  brought  about.  Nevertheless, 
it  is  the  most  obvious  of  such  stations  and  it  is  un- 
doubtedly traversed  constantly  by  impulses  which  have 
had  their  rise  in  the  eyes,  the  ears,  and  the  muscular 


118 


HUMAN    PHYSIOLOGY 


apparatus.  It  is  as  constantly  sending  out  impulses 
destined  to  modify  the  action  of  the  skeletal  muscles 
and  through  them  to  assist  in  preserving  equilibrium. 

The  Autonomic  System. — This  term  is  used  to  dis- 
tinguish that  part  of  the  efferent  nervous  system  trans- 
mitting impulses  from  the  central  axis  to  the  heart, 
the  glands,  and  the  smooth  muscle  of  the  entire  body. 


FIG.  27. — Above  is  shown  the  typic  motor  neuron  extending  from 
the  central  nervous  system  to  the  fibers  of  skeletal  muscle. 

Below  is  the  autonomic  type  of  path.  The  neuron  which  leaves  the 
central  nervous  system  does  not  span  the  whole  interval  but  ends  in 
synaptic  union  with  ganglion  cells.  These  in  turn  send  axons  to  the 
tissue  controlled — in  this  case  smooth  muscle. 

The  name  autonomic  is  nearly  equivalent  to  "  self- 
acting"  and  is  used  to  emphasize  the  contrast  which 
exists  between  these  mechanisms  and  the  skeletal 
muscles — the  latter  being  more  definitely  under  our 
control.  Autonomic  pathways  lead  away  from  the 
central  system  at  every  level  from  the  third  cranial 
nerve  to  the  most  posterior  portion  of  the  cord. 

We  have  seen  that  the  paths  from  the  brain  and  cord 


THE    BRAIN  119 

to  the  skeletal  muscles  are  represented  by  continuous 
fibers  spanning  the  interval.  In  the  autonomic  system 
the  case  is  otherwise.  Fibers  which  issue  from  the  brain 
and  cord,  bearing  impulses  destined  to  take  effect  in 
structures  other  than  skeletal  muscles,  do  not  actually 
extend  to  these  terminal  stations.  They  end  by  synapses 
against  nerve  cells  of  a  second  order  which  in  their  turn 
control  the  effectors  in  question.  These  distributing 
neurons  usually  have  their  cell-bodies  in  various  ganglia 
which  are  found  in  many  localities.  The  fibers  which 
run  from  the  central  nervous  system  to  the  way-stations 
are  called  pre-ganglionic,  those  which  relay  the  impulses 
to  the  end-organs  are  called  post-ganglionic.  It  appears 
to  be  the  rule  that  one  pre-ganglionic  neuron  stands  in 
relation  to  a  number  of  those  of  the  dependent  rank. 
It  seems  to  follow  that  impulses  sent  from  the  brain  and 
the  cord  to  the  ganglia  are  there  multiplied  and  diffused. 
There  can  hardly  be  the  precise  localizing  of  effect 
obtainable  in  the  command  of  skeletal  muscles. 

The  pre-ganglionic  fibers  proceeding  from  the  spinal 
cord  are  for  the  most  part  connected  with  ganglia  which 
are  placed  in  two  long  chains  to  the  right  and  left  of  the 
spinal  column  at  the  back  of  the  body  cavity.  These 
two  chains  of  ganglia  with  their  associated  axons  have 
been  known  as  the  sympathetic  system.  The  term  is 
an  odd  one  and  is  not  to  be  given  any  tinge  of  the 
psychologic  significance  naturally  coupled  with  it. 

The  functions  of  the  autonomic  system  may  be  sum- 
marized under  three  divisions:  First  we  have  those  of 
the  head  or  cranial  section.  Fibers  originating  here 
produce  effects  in  many  organs  of  the  trunk  as  well  as 
in  the  head.  They  can  bring  about  contraction  of  the 
pupil,  increase  of  curvature  of  the  crystalline  lens, 
changes  in  the  circulation  in  the  glands  and  skin  of  the 
head,  secretion  of  saliva,  and  slowing  of  the  heart. 
They  also  contract  the  bronchial  tubes,  cause  the  secre- 
tion of  gastric  juice,  and  reinforce  the  muscular  activity 
of  most  of  the  alimentary  canal. 


120  HUMAN    PHYSIOLOGY 

The  second  (thoracico-lumbar)  division  comprises 
all  the  autonomic  paths  leading  from  the  spinal  cord 
excepting  a  small  group  at  its  lower  end.  They  serve 
to  promote  the  contraction  of  most  of  the  blood-vessels, 
are  capable  of  suspending  the  activity  of  the  alimentary 
canal,  cause  secretion  of  sweat,  erection  of  the  hairs 
(or  goose-flesh  in  the  human  subject),  dilation  of  the 
pupil,  and  a  special  activity  of  the  adrenal  bodies  to  be 
referred  to  elsewhere.  They  accelerate  the  heart. 
In  the  case  of  this  organ,  the  alimentary  tract,  and  the 
iris  they  definitely  oppose  the  cranial  autonomic. 

The  small  group  of  paths  remaining  below  (sacral 
autonomic)  play  upon  the  rectum,  bladder,  and  re- 
productive organs. 

Many  functions  of  the  lower  parts  of  the  brain  which 
have  been  only  hinted  at  in  this  chapter  will  receive 
more  attention  later.  It  remains  in  the  present  treat- 
ment of  the  nervous  system  to  consider  the  significance 
of  the  cerebrum. 


CHAPTER  IX 
THE  BRAIN  (Continued)— THE  CEREBRUM 

The  importance  of  the  cerebrum  to  the  capacities 
of  animals  for  reaction  varies  within  the  widest  limits. 
We  shall  find  that  in  ourselves  it  is  without  question 
the  supreme  division  of  the  brain.  As  we  descend  in 
the  scale  of  vertebrate  life  its  place  is  less  and  less  im- 
pressive until  in  the  fish  it  serves  chiefly  to  connect  the 
organs  of  smell  with  the  brain-stem.  This  seems  a 
trifling  function  but  it  is  one  which  the  cerebrum  in 
all  cases  fulfils.  The  cerebrum  cannot  be  removed  for 
purposes  of  experiment  without  isolating  and  rendering 
useless  the  receptors  of  the  nose.  (We  have  seen 
already  that  none  of  the  cranial  nerves  beside  the  ol- 
factory connect  directly  with  the  cerebrum.) 

Decerebrate  Animals. — An  animal  from  which  the 
cerebrum  has  been  removed  is  termed  decerebrate. 
From  its  behavior  we  can  arrive  at  conclusions  regard- 
ing the  extent  to  which  the  cerebrum  figures  in  the 
life-processes  of  any  particular  species.  The  operation 
is  a  simple  one  in  the  cold-blooded  types  but  becomes 
increasingly  difficult  in  the  higher  forms.  It  has  been 
successfully  performed  upon  birds  and  even  dogs. 

Fish  are  but  little  affected  by  the  loss  of  this  part  of 
the  brain.  They  are  reported  to  behave  so  nearly  like 
their  mates  that  it  is  not  possible  to  identify  the  de- 
cerebrate  fishes  among  normal  ones  in  an  aquarium. 
They  show  the  same  preferences  for  certain  foods  that 
were  demonstrated  before  the  trial.  A  few  kinds  of 
fish — among  them,  sharks — are  said  to  become  notably 
inert,  but  close  observation  has  shown  that  this  is  owing 
to  the  loss  of  the  olfactory  organs  on  which  they  evi- 
dently depend  to  an  exceptional  degree. 

121 


122  HUMAN    PHYSIOLOGY 

Decerebrate  frogs  would  pass  for  perfect  animals  if 
the  inspection  were  not  quite  patient  and  conscientious. 
Those  who  have  studied  them  most  closely  tell  us  that 
they  are  rather  more  machine-like  or  less  ''spontaneous" 
than  intact  frogs.  They  are  liable  to  blunder  against 
obstacles  and  they  will  not  surely  save  themselves  from 
death  when  gradually  heated  in  a  pan  of  water.  This  is 
in  spite  of  the  fact  that  they  jump  with  vigor  when 
stimulated  in  other  ways.  It  is  claimed  that  a  frog 
which  has  learned  anything — how  to  escape  from  a 
certain  enclosure,  perhaps — will  lose  the  accomplish- 
ment when  decerebrated.  If  this  is  correct  it  is  of  the 
utmost  interest  for  it  foreshadows  the  special  property 
of  the  cerebrum  higher  in  the  scale. 

Those  animals  which  we  feel  warranted  in  calling 
" lower"  are  admirable  mechanisms  to  work  under  fixed 
conditions.  What  they  lack  is  the  power  to  profit  by 
experience.  We  cannot  say  that  this  is  absolutely  want- 
ing in  the  simplest  forms  but  it  is  more  and  more  evident 
as  we  consider  those  of  higher  rank.  In  precisely  the 
same  proportion  the  cerebrum  becomes  dominant  in 
the  nervous  system.  The  power  to  profit  by  experience, 
to  learn,  or  to  acquire  new  reactions  is  nearly  akin  to 
memory.  The  statement  that  the  cerebrum  is  the  organ 
of  associative  memory  can  be  strongly  defended. 

The  word  memory  is  likely  to  suggest  to  the  student 
conscious  recollection.  It  is  quite  as  correct  to  use  it 
objectively.  In  this  sense  the  memory  of  an  impres- 
sion is  a  modification  of  the  nervous  system  manifested 
through  the  fact  that  reactions  subsequent  to  the  event 
are  different  from  what  they  would  be  if  it  had  never 
occurred.  A  cat  when  left  alone  in  a  room  where  there 
is  meat  on  the  table  is  likely  to  jump  up  and  steal  it. 
After  certain  punishments  the  animal  may  refrain 
from  taking  the  food.  We  may  say  that  the  cat  re- 
members the  blows,  or  we  may  exclude  all  subjective 
ideas  and  say  that  the  nervous  system  has  been  so 
changed  that  it  does  not  react  as  it  did  at  first.  This 


THE   BRAIN  123 

latter  statement  is  not  to  be  challenged;  it  is  safer  than 
the  former  which  really  rests  on  our  imagining  ourselves 
in  the  place  of  the  cat. 

We  feel  very  skeptical  regarding  the  possibility  of 
disciplining  an  ant  so  that  it  will  not  approach  a  lump 
of  sugar.  The  fact  that  a  cat  can  be  trained  as  de- 
scribed while  an  ant  probably  cannot,  shows  just  what 
is  meant  by  the  "high"  as  contrasted  with  the  "low" 
type  of  nervous  system..  Wonderful  as  the  com- 
munity life  of  ants  and  bees  appears  to  us,  it  is  an 
inflexible  life  in  which  there  is  but  slight  differentiation 
of  individuals.  An  ant  which  has  passed  through  the 
several  stages  of  development  enters  into  the  life  of  the 
colony  with  a  certain  standard  equipment  for  the  part 
it  is  to  play.  It  probably  adds  scarcely  anything  to 
this  initial  equipment  as  a  result  of  its  contacts,  failures, 
and  successes.  It  does  not  come  to  have  any  individu- 
ality that  the  human  observer  can  discover.  Dogs 
and  cats,  on  the  contrary,  have  plastic  nervous  systems 
which  are  affected  by  countless  impressions  and  endow 
these  animals  with  well-marked  personalities. 

To  remove  the  cerebrum  is  to  rob  an  animal  of  what 
it  has  acquired  in  living  its  own  life.  What  remains 
is  the  common  heritage  of  the  race.  This  is  borne  out 
by  the  classic  experiments  upon  the  pigeon.  Early  in 
the  last  century  the  condition  of  this  bird  after  decere- 
bration  was  carefully  described.  Many  repetitions  of 
the  operation  have  modified  the  original  conclusions  in 
some  details  but  have  not  shaken  their  principal  lessons. 
The  decerebrate  pigeon  retains  striking  reflex  capacities 
but  can  hardly  be  thought  of  as  a  sentient  or  teachable 
being. 

The  bird  is  most  of  the  time  in  an  inert  state  suggestive 
of  dozing.  It  can  be  roused  by  positive  stimulation 
and  reacts  well  but  quickly  subsides  once  more.  For 
example,  if  it  is  tossed  into  the  air  it  will  fly  for  a  short 
distance.  The  direction  of  its  flight  is  probably  a 
matter  of  chance  but  it  will  avoid  obstacles  and  may 


124  HUMAN    PHYSIOLOGY 

swerve  toward  a  perch.  Having  alighted  it  sinks  into 
its  customary  moping  attitude.  Loud  sounds  cause 
it  to  stir,  but  sluggishly.  It  will  not  find  food  for  itself 
though  it  may  be  within  easy  reach.  It  does  not  re- 
spond to  the  presence  of  its  own  kind  nor  is  it  disturbed 
by  the  approach  of  its  natural  enemies.  Associative 
memory  is  gone  and  little  or  no  power  to  form  fresh 
associations  is  left. 

The  removal  of  the  cerebrum  from  a  mammal  was 
long  supposed  to  be  impossible.  It  was  triumphantly 
accomplished  about  twenty-five  years  ago  and  the  feat 
has  been  repeated  many  times.  In  the  first  successful 
decerebration  of  a  dog  the  brain  substance  was  cau- 
tiously washed  from  the  cavity  of  the  skull,  the  process 
being  completed  in  three  stages  with  long  intervals  to 
permit  recovery  from  the  shock.  The  dog  rallied  well 
and  was  kept  in  fair  condition  for  more  than  a  year, 
though  toward  the  last  its  strength  was  failing.  It 
was  finally  killed  and  the  autopsy  showed  that  cerebrum 
had  been  wholly  obliterated. 

This  dog  gave  the  same  general  impression  which  one 
derives  from  watching  a  decerebrate  pigeon.  The 
animal  was  mechanically  competent  but  idiotic.  It 
roved  about  the  laboratory,  making  detours  around 
objects  in  its  path  and  taking  the  same  pains  to  avoid 
patches  of  sunlight  on  the  floor.  It  chewed  and  swal- 
lowed food  which  was  brought  to  its  mouth  but  did 
not  seek  it.  It  would  snap  at  the  hand  of  anyone  who 
pinched  it  but  only  during  the  irritation ;  it  did  not  con- 
ceive of  such  a  one  as  an  enemy,  and  harbored  no  preju- 
dice against  him.  It  was  questionable  whether  it  ever 
learned  anything  by  experience  after  the  operations. 

The  human  cerebrum  is  very  large  in  proportion  to< 
the  rest  of  the  nervous  system  and  to  the  total  mass 
of  the  body.  This  coincides  with  the  evident  fact  that 
man  has  less  inherited  power  of  coordinated  action  than 
most  of  the  lower  animals  and  far  more  capacity  to 
develop  reactions  based  on  memory.  We  do  not  make 


THE    BRAIN  125 

deliberate  experiments  upon  the  human  brain,  but  nature 
makes  many  which  are  at  once  deplorable  and  in- 
structive. We  are  often  compelled  to  see  the  havoc 
wrought  by  hemorrhages,  tumors,  and  other  agencies 
which  injure  the  intricate  mechanisms  of  the  cerebrum. 
On  the  whole,  the  collected  data  confirm  the  belief  that 
we  have  here  the  physical  apparatus  of  intelligence. 

In  all  probability  the  human  cerebrum,  besides 
representing  an  advanced  type  of  outfit  for  recording 
individual  happenings,  has  taken  over  certain  functions 
which  in  the  dog  are  subserved  by  other  parts  of  the 
brain.  We  have  noted  the  use  of  the  eyes  in  main- 
taining equilibrium.  As  a  rule  this  is  a  subconscious 
service  and  we  have  seen  that  in  the  decerebrate  pigeon 
as  well  as  in  the  dog  it  continues  in  the  absence  of  a 
cerebrum.  It  is  not  likely  that  it  could  so  continue  in 
man.  When  certain  local  damage  is  suffered  by  the 
human  cerebrum  there  is  total  blindness  so  far  as  can 
be  determined;  no  subconscious  guidance  in  locomotion 
is  afforded  by  the  eyes. 

Quite  recently  there  was  published  a  description  of  a 
defective  child  that  lived  four  years  before  it  was 
happily  released  by  disease.  It  was  found  postmortem 
to  have  no  cerebrum  at  all,  the  space  being  filled  with 
fluid.  There  had  been  no  appreciable  progress  in  ac- 
quiring new  reactions  from  the  day  of  birth.  The  child 
seemed  blind  and  generally  lay  passive  as  though  sleep- 
ing. No  certain  token  of  consciousness  had  been  recog- 
nized. This  dreadful  case  lends  weight  to  the  usual 
view:  that  all  individual  gains  in  adjustment  to  the 
environment  are  conditioned  by  the  organization  of 
the  cerebrum. 

Localization. — Granting  that  intelligent  activities  have 
their  physical  accompaniments  in  the  cerebrum,  what 
can  be  said  of  the  distribution  of  these  processes?  This 
has  proved  a  fascinating  and  difficult  question.  For 
more  than  a  century  it  has  been  before  physiologists  and 
physicians.  In  1830  the  belief  in  precise  localization  of 


126 


HUMAN    PHYSIOLOGY 


function  was  very  widespread,  by  1860  it  had  been  re- 
placed by  skepticism  in  regard  to  any  such  topography. 
In  1890  a  new  doctrine  of  localization,  almost  unrelated 
to  the  old,  had  gained  general  acceptance.  At  present 
there  is  another  reaction;  the  tone  of  most  writers  is 
cautious  and  conservative.  We  can  deal  only  very 
briefly  with  these  pendular  movements  of  scientific 
thought. 

The  old  teaching  is  remembered  under  the  name  of 


FIG.  28. — The  human  cerebrum  is  sketched  from  the  left  side. 
The  crosses  are  sprinkled  on  the  area  from  which  the  muscles  of  the 
right  half  of  the  body  appear  to  be  governed.  This  area  extends  out  of 
sight  over  the  top  of  the  brain  and  dips  into  the  fissure  between  the  two 
hemispheres. 

Phrenology.  The  early  advocates  of  localization  busied 
themselves  with  attempts  to  correlate  the  character  and 
accomplishments  of  men  with  the  forms  of  their  heads, 
assuming  rather  rashly  that  the  shape  of  the  brain  can  be 
accurately  judged  from  the  contours  of  the  skull.  They 
came  to  believe  that  numerous  subdivisions  of  the 
cerebrum  must  exist,  each  standing  for  a  mental  char- 
acteristic. The  idea  is  that  of  so-called  "  bumps." 
Phrenologic  diagrams  show  how  far  their  makers  believed 
the  analysis  could  be  carried.  The  inferences  drawn 


THE   BRAIN  127 

became  more  and  more  far-fetched  until  the  school  fell 
wholly  into  disrepute. 

The  conception  of  a  cerebrum  having  its  surface 
plotted  in  little  areas  concerned  with  specific  functions 
gave  way  for  a  time  to  the  view  that  there  is  no  distinction 
between  the  duties  of  its  different  parts.  It  was  com- 
pared with  the  liver,  a  large  organ  having  several  simul- 
taneous activities.  A  particular  lobe  of  the  liver  is  not 
supposed  to  have  a  single  service  but  rather  to  participate 
in  all  the  work  of  the  organ  as  a  whole.  So  it  came  to  be 
thought  of  the  brain  that  cerebral  functions  are  not 
local  but  diffuse.  To  destroy  a  part,  it  was  held,  would 
not  suppress  any  one  power  but  would  weaken  all. 

When  new  evidence  of  localization  in  the  cerebrum 
began  to  be  obtained  the  emphasis  was  quite  different 
from  that  which  had  prevailed  in  the  days  of  phrenology. 
Less  was  said  of  the  correlation  of  brain  and  mind  and 
more  of  the  correlation  of  brain  and  body.  The  first 
special  areas  to  be  defined  were  those  which  we  call 
motor,  regions  of  the  cortex  which  appear  to  have  a 
closer  connection  with  the  skeletal-  muscles  than  can  be 
claimed  for  other  portions. 

The  Motor  Areas. — These  are  distinguished  by  the 
fact  that  electric  stimulation  applied  anywhere  within 
their  boundaries  causes  movements  of  various  parts  of 
the  body.  The  muscular  responses  occur  chiefly  on  the 
opposite  side  as  already  implied.  The  motor  areas 
have  been  carefully  mapped  for  the  cat,  the  dog,  and  the 
ape.  In  the  last-named  animal  the  general  appearance 
of  the  brain  is  very  like  the  human  and  there  is  no  doubt 
that  in  the  cerebrum  of  man  there  are  motor  areas  in  a 
corresponding  position.  A  diagram  will  show  how  they 
are  placed.  Cerebral  localities  are  referred  to  under  the 
names  of  the  bones  of  the  skull  which  lie  over  them  and 
these  areas  are  said  to  be  in  the  frontal  region  along  the 
border  of  the  parietal.  Roughly,  they  may  be  said  to 
extend  upward  from  the  ear  to  the  top  of  the  head. 

Microscopic  study  of  the  brain  shows  that  the  surface 


128  HUMAN    PHYSIOLOGY 

gray  matter  of  the  motor  regions  contains  many  large 
nerve  cells,  of  a  type  called  pyramidal,  from  which  the 
axons  of  nerve  fibers  pass  inward.  The  fibers  which 
thus  have  their  origin  in  the  motor  areas  become  con- 
densed into  well-defined  bundles  which  can  be  recognized 
at  each  successive  level  in  the  brain-stem.  In  the  medulla 
most  of  these  fibers  sweep  across  into  the  opposite  half 
of  the  nervous  system  and  descend  the  spinal  cord  to 
connect  with  the  motor  cells  in  its  gray  core.  It  will  be 
noted  that  a  fiber  from  the  cortex  never  reaches  a  muscle; 
it  plays  upon  a  cluster  of  cells  of  a  lower  order  and  the 
contractile  elements  are  governed  by  these.  Something 
like  this  has  been  seen  to  be  true  of  the  respiratory 
center  in  the  medulla. 

When  the  motor  areas  in  man  are  extensively  damaged, 
or  when  the  fibers  carrying  impulses  down  the  axis 
from  these  areas  are  interrupted,  a  disabling  paralysis 
results.  This  is  to  be  contrasted  with  the  coordinating 
powers  of  the  dog  which  can  lose  not  merely  the  motor 
regions  of  the  cerebrum  but  the  whole  of  that  division 
and  still  balance  and  walk.  We  have  here  another  il- 
lustration of  the  concentration  of  functions  in  the  cere- 
brum of  the  higher  forms  and  the  reduction  of  capacity 
in  the  cord  and  brain-stem. 

Movements  which  we  call  voluntary  are  assumed  to 
be  preceded  by  processes  in  the  cerebral  motor  areas. 
In  all  probability  this  is  equally  true  of  many  move- 
ments which  we  class  as  involuntary.  The  line  of 
demarcation  is  arbitrary  and  of  little  value.  Even 
when  an  act  is  as  clearly  as  possible  deliberate  we  can- 
not say  that  the  process  originated  here  or  there  in  the 
cerebrum.  The  excitation  of  a  motor  spot  is  presum- 
ably to  be  referred  to  some  other  part  of  the  brain  and 
as  we  attempt  to  trace  the  action  to  its  source  we  are 
baffled  and  left  unsatisfied.  We  are  forced  toward  the 
conclusion  that  the  ultimate  cause  of  every  movement 
is  to  be  sought  on  the  afferent  side  of  the  nervous  sys- 


THE   BRAIN 


129 


tern.     But  we  rebel  vigorously  at  the  suggestion  that 
there  is  no  such  thing  as  choice  or  freedom. 

Sensory  Areas. — As  the  motor  regions  of  the  cere- 
brum are  regarded  as  places  from  which  impulses  take 
their  departure  to  affect  subordinate  mechanisms,  so 
there  are  in  the  cortex  areas  within  which  sensory  cur- 


FIG.  29. — The  upper  figure  represents  the  left  hemisphere  from  out- 
side, that  is,  from  the  left.  The  lower  figure  shows  the  internal  or 
mesial  aspect  of  the  right  hemisphere  from  the  left  side.  Areas  usually 
claimed  to  possess  special  relations  are  marked  .as  follows:  H,  hearing; 
V,  vision;  Sm,  smell;  Sp,  speech. 

rents  are  received.  As  the  cells  of  the  cerebrum  never 
connect  directly  with  the  fibers  of  skeletal  muscle,  so, 
the  receptors  of  the  body  never  connect  directly  with 
the  cortex.  At  least  one  relay  is  made  on  each  sensory 
path  and  usually  rnqre  than  one.  We  are  best  ac- 
quainted with  the  relations  between  the  eyes  and  the 
brain. 


130  HUMAN    PHYSIOLOGY 

Each  retina  is  a  cup-shaped  structure  functionally 
subdivided  like  a  mosaic  into  a  vast  number  of  minute 
areas,  every  one  having  its  own  connection  with  the 
optic  nerve.  The  fibers  of  the  optic  nerves  can  be  fol- 
lowed, as  we  have  seen,  into  the  part  of  the  brain- 
stem  which  is  just  below  the  cerebrum.  There  they 
come  into  relation  with  nerve  cells  through  which  re- 
flexes can  be  produced  and  with  other  cells  which  trans- 
mit impulses  to  the  cerebrum.  It  is  not  supposed  that 
any  intelligent  use  of  the  visual  power  can  be  based  on 
the  employment  of  the  brain-stem  by  itself.  We  believe 
that  the  cerebrum  must  be  involved. 

The  part  of  the  cortex  to  which  the  impulses  of  visual 
origin  are  first  sent  is  as  far  from  the  eyes  as  possible, 
the  rear  or  occipital  region  of  the  cerebrum.  The  area 
most  certainly  used  is  a  portion  of  the  surface  where 
the  right  and  left  hemispheres  are  in  contact,  the  sides 
of  the  deep  cleft  which  is  between  them.  Injuries  here 
have  caused  blindness  in  many  subjects.  We  know 
that  the  central  part  of  the  retina  has  superior  usefulness 
in  seeing  and  it  is  probable  that  its  connections  are  cor- 
respondingly extensive.  When  sight  is  recovered  after 
its  temporary  loss  through  pressure  on  the  visual  cortex, 
the  patient  may  at  first  see  only  in  a  very  small  central 
field  and  this  may  gradually  widen  as  the  improvement 
continues. 

After  learning  of  the  crossed  relation  between  the 
cerebrum  and  the  muscles  one  may  expect  to  hear  that 
the  right  eye  is  connected  with  the  left  side  of  the  brain. 
The  actual  arrangement  is  less  simple.  The  right  half 
of  the  right  retina,  approximately,  and  the  right  half 
of  the  left  retina  also,  have  relations  with  the  right  side 
of  the  brain.  The  images  on  the  right  halves  of  the 
retinas  are  those  of  objects  toward  the  left  of  the  ob- 
server. It  follows  that  destruction  of  the  visual  cortex 
on  the  right  side  of  the  cerebrum  will  result  in  loss  of 
vision  in  the  left  half  of  the  field.  Strictly  speaking, 
it  is  said  that  the  loss  in  such  cases  is  a  little  less  than 


THE   BRAIN  131 

half,  the  central  element  being  preserved  entire  instead 
of  being  cut  through  on  a  meridian.  This  is  inter- 
preted as  signifying  a  reduplication  of  the  representa- 


FIG.  30. — The  relation  between  the  retinas  and  the  brain.  The 
right  half  of  each  retina  is  heavily  scored  and  will  be  seen  to  be  con- 
nected with  the  right  occipital  region  through  intermediate  centers. 

tion  of  this  superior  central  area  in  the  two  occipital 
regions. 

Impulses  arriving  from  the  part  of  the  ear  which 
has  to  do  with  hearing  traverse  the  medulla  and  ascend 


132  HUMAN    PHYSIOLOGY 

by  relays  through  the  remaining  portion  of  the  brain- 
stem,  reaching  the  cortex  at  last  in  what  is  called 
the  temporal  lobe.  The  majority  of  impulses  from  the 
right  ear  are  said  to  be  brought  to  the  cortex  on 
the  opposite  side  although  a  fraction  seem  to  reach  the 
station  on  the  side  of  their  origin.  Impulses  from  the 
nose,  adapted  to  cause  sensations  of  smell,  are  distributed 
near  their  point  of  entrance  into  the  cerebrum  on  its 
under  surface.  There  is  some  uncertainty  as  to  the 
cerebral  localization  of  taste.  Behind  the  motor  areas 
in  the  ape,  and  probably  in  man,  there  are  strips  of  the 
cortex  within  which  many  sensory  impulses  from  the 
body  in  general  seem  to  be  focused. 

Association  Areas. — As  we  have  pictured  the  cere- 
brum it  has,  on  each  side,  several  areas  of  a  receptive 
function  and  one  well-marked  area  from  which  impulses 
take  their  departure  to  initiate  movements.  If  an 
animal  is  to  behave  in  a  normal  manner,  that  is,  to  be 
guided  by  its  circumstances  in  what  it  does,  there  must 
be  connecting  links  between  the  receptive  and  the  dis- 
charging stations.  The  higher  the  organization  the 
more  devious  and  difficult  to  trace  are  these  connecting 
links.  There  may  be  short  cuts  between  sensory  and 
motor  regions  insuring  certain  reactions  of  a  reflex  type 
but  many  of  the  ways  of  communication  are  most  in- 
direct. It  is  supposed  that  they  are  made  by  inter- 
mediate portions  of  the  cortex  to  which  we  give  the 
name  of  the  association  areas.  These  are  not  united 
by  fibers  with  the  brain-stem  but  only  with  each  other 
and  with  the  sensory  and  motor  fields.  Fibers  which 
run  between  the  cortex  and  the  brain-stem  or  cord  are 
called  projection  fibers;  they  connect  higher  with  lower, 
or  lower  with  higher,  levels.  Fibers  which  run  from 
point  to  point  in  the  cortex  are  called  association  fibers. 
According  to  these  definitions  the  association  areas  have 
no  projection  fibers. 

It  is  natural  to  consider  the  association  centers  as 
the  highest  of  all  in  the  character  of  their  service.  They 


THE   BRAIN  133 

are  farther  removed  from  both  muscles  and  sense- 
organs.  They  are  known  to  be  late  in  their  develop- 
ment. The  more  extended  and  subject  to  change  the 
paths  from  simple  sensory  to  primitive  motor  centers 
the  more  variation  we  may  expect  to  observe  in  the 
conduct  of  an  animal — the  more  spontaneous  and  the 
less  machine-like  it  will  appear.  Every  such  path  is 
supposed  to  lie  through  association  areas.  In  the 
striking  out  of  these  paths  there  is  the  greatest  possibility 
for  individual  divergence. 

In  the  human  brain  one  of  the  great  association  areas 
lies  in  the  frontal  region  and  another  in  the  parietal. 
There  are  doubtless  many  other  parts  of  the  cortex, 
of  less  extent  than  these,  which  have  the  same  char- 
acteristic: that  is,  absence  of  direct  connection  with 
the  brain-stem.  Disease,  when  limited  to  such  parts 
of  the  cortex,  does  not  produce  clear  and  definite  symp- 
toms, like  paralysis,  blindness,  or  deafness.  It  is  more 
likely  to  result  in  indefinite  but  serious  loss  of  intelligence. 
Sometimes  we  hear  of  a  wonderful  preservation  of  all 
the  faculties  in  spite  of  gross  injuries  by  gunshot  wounds 
or  otherwise.  When  we  consider  the  fatal  effects  com- 
monly following  brain  injuries  we  must  bear  in  mind 
that,  when  the  skull  is  penetrated,  pressure  and  dis- 
placements of  tissue  may  occur  at  some  distance  from 
the  apparent  seat  of  the  damage. 

When  we  compare  ourselves  with  the  lower  animals 
we  are  apt  to  think  most  of  our  psychic  life.  Beyond 
question  we  are  right  in  doing  so  but  the  physiologist, 
anxious  to  keep  objective  standards  so  far  as  he  may, 
will  say  that  we  are  distinguished  by  our  varied  reac- 
tions. The  brain  of  man  is  adapted  to  execute  these 
responses,  to  blend  them,  and  also  to  suppress  certain 
ones  in  favor  of  others.  This  last  is  a  function  of  the 
utmost  significance. 

Cerebral  Inhibition. — If  we  reflect  a  little  upon  what 
constitutes  strength  of  character  we  realize  that  the 
man  we  admire  does  not  do  various  things  which  a 


134  HUMAN    PHYSIOLOGY 

weaker  and  less  disciplined  individual  would  do.  We 
say  that  he  exhibits  self-control.  He  does  not  fly  into 
a  passion  on  moderate  provocation;  he  does  not  quail 
when  in  danger.  How  shall  we  express  these  facts  in 
the  language  of  physiology?  We  shall  probably  choose 
to  say  that  primitive  reflexes  have  been  displaced  by 
reactions  more  recently  acquired.  The  dominant  re- 
actions are  those  for  which  the  indirect  paths  by  way 
of  the  association  areas  are  necessary.  We  say  that 
the  more  elementary  tendencies  are  inhibited.  This 
is  an  obvious  principle  in  matters  of  good  breeding  as 
well  as  virtue.  While  it  is  eminently  a  feature  of  human 
life  it  is  not  wholly  wanting  in  the  animals  that  we  feel 
to  be  real  comrades.  It  is  signally  displayed  by  the 
dog  that  refrains  from  biting  the  child  that  torments  it. 

Language. — One  of  the  most  remarkable  of  human 
powers  is  that  of  uttering  and  comprehending  words. 
A  baby  is  guided  by  spoken  words  much  earlier  than  is 
commonly  supposed  and  long  before  it  reproduces  the 
words  that  it  hears.  In  course  of  time  it  learns  to  talk. 
Later  the  child  is  taught  to  read  and  to  write,  additional 
modes  of  making  use  of  language.  When  the  nervous 
system  is  fully  developed,  every  spoken  or  written 
word  is  a  stimulus  with  a  definite  power  to  affect  it  and 
to  secure  reactions  from  it.  Brain  disease  often  per- 
verts or  abolishes  the  use  of  language.  Sometimes  it 
is  the  ability  to  speak  that  is  interfered  with,  sometimes 
the  capacity  to  act  under  the  guidance  of  words  spoken 
by  others.  In  the  latter  case  we  say  that  the  patient 
does  not '  'understand  "  what  he  hears.  This  is  a  psycho- 
logic inference;  what  we  are  sure  of  is  that  his  conduct 
is  not  determined  by  it. 

There  is  a  small  region  in  the  frontal  lobe,  on  the  left 
side  in  most  of  the  subjects  studied,  which  is  reputed 
to  be  a  speech  center.  It  is  close  to  the  part  of  the  general 
motor  area  from  which  the  muscles  of  the  vocal  organs 
are  supplied.  It  will  be  noted  that  these  muscles  are 
not  used  exclusively  in  speech.  They  are  employed 


THE   BRAIN  135 

in  mastication,  swallowing,  coughing,  and  many  other 
acts.  They  are  also  contracted  when  simple  animal 
outcries  are  made.  The  power  of  speech  is  much  more 
than  the  power  to  command  these  muscles;  it  is  the  power 
to  make  them  express  thought.  When  the  alleged 
center  suffers  from  local  impairment  through  disease 
the  disorder  which  results  is  not  a  paralysis  but  a  loss 
of  the  ability  to  make  thought  vocal.  This  is  what 
we  mean  by  motor  aphasia. 

Of  course  all  the  evidence  that  can  be  gathered  for 
the  existence  of  a  speech  center  is  based  upon  post- 
mortem discoveries  of  abnormal  conditions  at  this 
place  when  disorders  of  language  have  developed  during 
life.  The  correlation  of  disease  with  loss  of  coherent 
speech  has  indicated  that  in  left-handed  persons  the 
control  of  speech  is  dependent  on  the  right  half  of  the 
brain,  that  is,  on  the  side  which  also  presides  over 
the  skilled  hand.  So  in  the  average  individual  who  is 
right-handed  the  mechanisms  for  speech  and  for  the 
more  competent  hand  are  again  associated  but  on  the 
left  side.  Claims  for  the  existence  of  centers  for  read- 
ing and  writing  and  for  the  interpretation  of  words  heard 
have  been  strongly  urged.  Further  detail  would  be  out 
of  place  in  a  book  of  this  size. 

Summary. — The  cerebrum,  like  other  parts  of  the 
brain,  is  a  place  where  impulses  are  received  and  so 
applied  as  to  cause  other  impulses  to  be  sent  out.  But, 
far  beyond  any  other  division,  it  is  influenced  by  the 
passage  of  impulses  through  it.  It  is  not  so  much 
organized  by  inheritance  as  by  experience.  Its  capacities 
at  birth  are  potential;  the  manner  of  their  realization 
cannot  be  foretold.  In  old  age  it  has  registered  the 
story  of  a  life;  it  is  the  material  basis  of  memory  and 
so  of  every  individual  attainment.  The  suggestions 
relating  to  personal  hygiene  will  be  given  a  place  in 
another  chapter. 


CHAPTER  X 
SENSATIONS  AND  THE  SENSE-ORGANS 

We  have  spoken  in  a  general  way  of  the  receptors 
of  the  body  as  sources  of  nerve-impulses  which  are  con- 
veyed to  the  central  nervous  system  and  bring  about 
reflexes  which,  as  a  rule,  are*  clearly  valuable  to  the 
organism.  Now  we  must  give  some  attention  to  the 
states  of  our  consciousness  which  apparently  result 
from  the  stimulation  of  receptors.  Our  subject  is  that 
of  the  sensations  and  it  is  one  common  to  physiology 
and  psychology. 

We  cannot  take  up  this  matter  from  a  metaphysical 
standpoint.  On  the  physical  side  a  sensation  is  the 
accompaniment  of  a  certain  process  in  the  brain  which 
we  usually  assume  to  have  been  induced  by  external 
forces  acting  upon  the  receptors.  One  of  the  very  first 
things  to  be  insisted  upon  is  that  sensations  may  be 
experienced  when  the  external  stimuli  are  absent,  pro- 
vided the  brain  process  takes  place.  This  is  often 
the  case,  we  may  suppose,  in  our  dreams.  Sensations 
that  do  not  have  any  external  cause  demonstrable  to 
other  people  are  called  hallucinations.  The  more  one 
studies  the  nervous  system  the  more  the  wonder  grows 
that  we  are  not  often  deceived  in  regard  to  the  origin 
of  our  sensory  experiences. 

A  question  that  has  to  be  dealt  with  at  the  outset  is 
whether  or  not  nerve-impulses  are  all  alike.  It  is  plain 
that  they  are  most  unlike  in  their  effects.  Those  that 
go  to  the  skeletal  muscles  throw  them  into  contraction, 
others  running  to  the  glands  set  them  to  secreting,  still 
others  led  to  the  heart  may  restrain  it  from  beating. 
On  the  afferent  side. the  impulses  that  enter  the  brain 

136 


SENSATIONS    AND    THE    SENSE    ORGANS          137 

by  the  optic  pathway  give  rise  to  visual  sensation,  while 
those  from  the  upper  part  of  the  nose  are  responsible 
for  olfactory  impressions.  One  would  be  inclined  to 
say  that  the  impulses  must  be  of  the  most  widely  varied 
character.  It  is,  therefore,  a  surprise  to  the  student 
to  hear  that  most  physiologists  have  adhered  to  the 
view  that  they  are  essentially  uniform  in  all  nerves. 

How  can  we  account  for  the  facts  before  us  without 
admitting  that  the  impulses  can  differ  in  kind?  It 
should  be  said  frankly  that  this  may  be  impossible,  but 
we  always  prefer  a  simpler  conception  to  one  which  is 
more  involved  unless  we  are  compelled  to  adopt  the 
latter.  If  nerve-impulses  are  all  alike,  our  emphasis 
must  be  upon  the  places  to  which  they  go  rather  than 
upon  the  currents  themselves.  We  shall  find  that  there 
are  analogies  to  help  us  in  the  attempt  to  do  this. 

The  lighting  of  a  gas  jet  and  the  ringing  of  a  bell  are, 
perhaps,  as  different  as  the  movement  of  a  muscle  and 
the  act  of  secretion  performed  by  a  gland.  Both  may 
be  caused  by  electric  currents  produced  in  similar 
batteries  but  led  through  different  and  suitable  fix- 
tures. No  one  would  argue  that  the  current  used  to 
light  the  gas  had  any  specific  gas-lighting  virtue  in 
itself;  it  could  as  well  do  something  else.  This  is  parallel 
with  what  is  known  as  the  Mullerian  doctrine  in  re- 
gard to  the  nervous  system:  that  nerve-impulses  vary 
only  in  intensity — not  in  kind — and  that  they  produce 
one  effect  or  another  according  to  the  particular  struc- 
tures on  which  they  finally  act. 

The  impulses  in  the  fibers  of  the  optic  nerve  have  been 
assumed  to  be  of  the  same  order  as  those  in  the  auditory. 
Upon  this  assumption  it  has  been  suggested  that  if 
impulses  having  their  origin  in  the  retina  under  the 
influence  of  light  could  be  switched  over  into  the  path 
from  the  organ  of  hearing  and  so  arrive  at  the  hearing 
centers  of  the  cerebrum  anything  ordinarily  visible 
would  be  audible;  one  could  listen  to  lights  and  colors. 
There  is  another  consequence  of  the  Mullerian  principle 


138 


HUMAN    PHYSIOLOGY 


FIG.  31. —The 
"crazy  bone." 
The  ulnar  nerve 
is  exposed  to 
rather  frequent 
shocks  near 
the  point  of 
the  elbow  (c). 
Impulses  run- 
ning up  to  the 
brain  as  a  result 
of  such  acci- 
dents produce 
sensory  effects 
which  are  much 
the  same  as 
though  they 
had  traversed 
the  whole  length 
of  the  hand. 


that  is  more  easily  tested.  If  impulses 
are  always  the  same  then,  no  matter  what 
means  may  have  been  employed  to  start 
them,  invariable  effects  will  be  produced 
by  the  stimulation  of  a  selected  path.  If 
all  the  fibers  in  the  optic  nerve  have  such 
connections  with  the  brain  as  to  arouse 
sensations  of  light  then  we  may  excite  the 
optic  nerve  in  an  unusual  way  and  still 
obtain  the  only  kind  of  sensation  proper 
to  this  collection  of  fibers.  This  seems 
to  be  very  nearly  true. 

We  have  testimony  to  the  fact  that  cut- 
ting this  great  nerve  does  not  give  pain 
but  the  sensation  of  a  bright  flash.  Pres- 
sure on  the  eyeball  will  produce  spots  or 
circles  of  color.  If  it  is  objected  that 
there  is  also  the  sense  of  pressure  it  can 
be  replied  that  a  different  nerve  from  the 
optic  is  known  to  be  concerned  in  this 
reaction.  Electric  stimulation  of  the  end- 
ings of  the  olfactory  nerve  is  said  to  cause 
an  odor. 

One  result  of  the  alleged  uniformity  of 
nerve-impulses  is  that  we  have  no  way  to 
tell  whether  they  come  from  the  distant 
endings  of  a  nerve  or  from  some  point  in- 
termediate between  these  endings  and  the 
centers.  This  is  simply  illustrated  by  the 
common  mishap  of  striking  the  elbow  a 
sharp  rap.  The  nerve  that  passes  over 
the  bone  at  that  joint  is  mechanically  ex- 
cited and  the  impulses  which  ascend  it 
have  the  same  effect  as  though  they  had 
come  all  the  way  from  the  fingers.  We 
say  that  the  fingers  tingle  because  we  are 
more  used  to  sensations  caused  through 

of  the  fibers  from  their  receptors  in  the  shaded  region 


SENSATIONS    AND    THE    SENSE-ORGANS  139 

their  receptors  than  to  those  produced  by  the  abnormal 
method. 

A  closely  related  experience  is  that  of  the  victim  of 
an  amputation.  He  describes  minutely  pains  and  other 
feelings  which  seem  to  come  from  the  lost  limb.  We 
have  only  to  reflect  that  all  the  fibers  which  formerly 
conducted  impulses  from  the  missing  part  are  still 
present  in  the  stump  and  if  they  are  stimulated  by 
reason  of  temporary  conditions  there  they  will  give  rise 
to  the  old,  familiar  sensations.  The  engineer  of  a  steam-, 
boat  cannot  tell  whether  his  gong  has  been  rung  from 
the  pilot  house  or  from  some  other  station.  We  are 
usually  saved  from  confusion  by  an  important  property 
of  the  afferent  system:  that  nerve  fibers  are  much  more 
resistant  to  stimulation  along  their  course  than  at  their 
specialized  receptive  terminations. 

The  Classification  of  Sensations. — The  five  senses 
ordinarily  recognized  do  hot  satisfy  the  requirements  of 
an  inclusive  enumeration.  They  are  all  of  the  class 
which  the  physiologist  calls  special.  He  sets  over  against 
them  another  group,  the  common  or  general  sensations, 
in  which  he  places  those  which  seem  to  have  their 
causation  within  the  body.  Special  sensations  are 
those  referred  to  the  external  world.  Examples  of 
common  sensations  are  hunger,  thirst,  fatigue,  nausea, 
and  many  kinds  of  pain.  One  is  struck  with  the 
prominence  of  disagreeable  feelings  in  this  class  and 
this  may  be  connected  with  the  fact  that  they  usually 
stand  for  conditions  that  call  for  readjustment.  Be- 
cause these  sensations  are  unpleasant  we  are  prompted 
to  take  measures  for  their  abatement  and  the  interest 
of  the  organism  is  served. 

There  are  some  sensations  which  lie  on  the  border 
line  between  the  general^  and  the  special  orders.  In 
explaining  how  the  equilibrium  is  preserved  we  men- 
tioned the  afferent  impulses  from  the  musculature. 
These  are  mostly  applied  subconsciously  to  produce 
appropriate  reflexes  but  this  is  not  the  whole  story. 


140  HUMAN    PHYSIOLOGY 

They  have  to  do  with  those  sensations  which  are  called 
kinesthetic,  those  feelings  which  we  have  respecting 
our  posture,  and  our  ability  to  realize  our  own  move- 
ments. One  need  not  look  at  the  extremities  to  see 
how  they  are  disposed;  there  is  sensory  evidence  of 
one's  position  at  all  times. 

Our  temperature  sensations,  also,  lie  in  a  class  inter- 
mediate between  the  varieties  which  are  clearly  special 
and  those  which  are  necessarily  general.  If  a  small 
.object  touches  the  skin  we  say  that  the  object  is  hot  or 
cold;  we  do  not  consider  that  the  patch  of  skin  has  taken 
the  temperature  of  the  thing  in  contact-*  with  it.  But 
if  the  whole  surface  of  the  body  is  warmer  than  usual 
we  may  say  either  that  the  room  is  warm  or  that  we 
are. 

Under  the  head  of  special  sensations  we  have  those 
produced  by  the  stimulation  of  the  skin  and  those 
pertaining  to  taste,  smell,  hearing,  and  vision.  In 
other  words,  these  are  the  five  senses  with  the  qualifica- 
tion that  what  is  called  touch  is  multiple.  It  is  to  be 
resolved  into  pressure,  warmth,  cold,  and  probably 
surface  pain.  Surprise  may  be  expressed  that  warmth 
and  cold  should  be  separately  mentioned.  But  we 
have  pointed  out  elsewhere  that  the  physical  conception, 
according  to  which  cold  is  merely  less  heat,  cannot  be 
applied  to  such  a  system  as  this.  Cold  stimulates  nerve- 
endings  as  truly  as  does  warmth  and  with  equally  positive 
results. 

If  all  nerve-impulses  are  alike  how  can  we  explain 
the  three  or  four  distinct  sensory  capacities  of  the  skin? 
There  is  only  one  resort — to  assume  that  there  are  as 
many  separate  sets  of  receptors  as  there  are  sensory 
qualities.  This  is  the  usual  belief.  Close  study  of  the 
responses  to  be  obtained  from  the  skin  has  shown  that 
there  are  minute  areas  which  are  capable  of  furnishing 
one  sensation  and  no  more.  Thus  there  are  pressure 
points,  warm  points,  cold  points,  and  pain  points,  ac- 
cording to  the  standard  description.  An  area  with  one 


SENSATIONS    AND    THE    SENSE-ORGANS          141 

and  only  one  sensory  property  presumably  has  just 
beneath  it  a  receptor  for  one  kind  of  stimulation.  The 
capacity  for  sensation  which  we  can  demonstrate  for 
the  skin  holds  with  minor  differences  for  the  lining  of 
the  mouth.  In  the  esophagus  we  have  a  persistence 
of  the  response  to  temperature  changes  but  a  practical 
loss  of  the  other  feelings. 

Liminal  Distance. — A  general  idea  of  the  relative 
refinement  of  the  pressure  sense  in  various  parts  of  the 
body  may  be  secured  by  finding  how  far  apart  two  blunt 
points  must  be  placed  so  that  when  they  are  applied 
to  the  skin  they  shall  be  felt  as  two  rather  than  one. 
The  interval  necessary  for  any  part  of  the  surface  of 
the  body  is  called  the  liminal  distance.  It  is  least  on 
the  tip  of  the  tongue  where  two  points  need  be  separated 
by  only  J/£5  inch  to  be  felt  distinctly.  The  dis- 
tance is  about  twice  as  great  on  the  finger  tips.  This 
explains  why  a  cavity  in  a  tooth  feels  much  larger  when 
the  tongue  is  presseed  into  it  than  when  it  is  examined 
with  the  finger.  Some  portions  of  the  skin  are  in- 
credibly deficient  in  discrimination;  on  the  back  it  may 
be  necessary  to  touch  points  2  inches  apart  to  have  their 
separate  nature  apparent  to  the  subject. 

Taste. — The  receptors  for  this  sense  are  mostly  on 
the  tongue.  They  are  stimulated  by  many  substances 
which  must  be  in  solution  to  have  an  effect.  There  are 
probably  but  four  distinct  kinds  of  taste:  sweet,  sour, 
bitter,  and  salt.  The  countless  other  qualities  which 
we  detect  in  our  food  are  compounded  of  odors,  con- 
tacts, temperature  impressions,  and  in  some  cases  an  ele- 
ment of  irritation  verging  on  pain.  There  are  probably 
four  sets  of  fibers  leading  from  the  tongue  to  the  brain 
and  their  endings  have  a  somewhat  characteristic 
distribution. 

Sweet  substances  are  most  stimulating  to  the  tip  of 
the  tongue.  (Observe  the  happy  child  boring  into  the 
interior  of  a  bonbon  with  that  part  of  the  little  member). 
Salt  affects  rather  strongly  the  edges  of  the  tongue  and 


142  HUMAN    PHYSIOLOGY 

anything  which  is  at  all  bitter  is  doubly  so  when  it 
passes  over  the  root  at  the  moment  of  swallowing. 
The  acid  appreciation  is  more  nearly  uniform  in  different 
places. 

It  has  been  proved  that  taste,  or  a  sense  closely  akin 
to  it,  is  mediated  by  receptors  in  the  skin  of  certain 
fishes.  A  bit  of  food  held  near,  but  not  touching  the 
skin  of  a  catfish,  far  back  toward  the  tail,  may  cause  the 
animal  to  whirl  about  and  seize  the  morsel.  In  view 
of  the  fact  that  taste  is  a  sense  wrought  upon  by  matters 
in  solution  it  is  not  surprising  that  an  animal  living  in 
a  liquid  medium  should  have  this  sense  represented 
upon  other  surfaces  than  that  of  the  mouth  cavity. 

Smell. — When  we  pass  from  taste  to  smell  we  recog- 
nize that  we  have  taken  an  important  step.  Taste, 
like  the  sensations  which  can  be  evoked  through  the 
receptors  of  the  skin,  requires  the  actual  contact  of  the 
stimulating  substance.  This  may  be  true  of  smell  also 
in  the  sense  that  portions  of  the  odorous  material  must 
be  brought  to  the  nerve-endings  high  in  the  nose,  but 
our  reference  or  projection  in  this  case  is  to  distant 
sources  of  the  radiation.  When  one  stands  at  the  edge 
of  a  pond  and  enjoys  the  fragrance  of  the  waterlilies 
one  does  not  think  that  some  portion  of  the  flowers  is 
in  the  nose.  On  the  other  hand,  when  a  crystal  of  salt 
is  tasted  one  does  not  refer  the  sensation  to  the  contents 
of  the  box  from  which  the  sample  has  been  taken.  The 
difference  in  the  mental  attitude  is  very  interesting. 

Endeavors  to  classify  odors  have  not  been  particularly 
successful.  The  number  of  types  seems  large,  and 
different  judges  can  hardly  agree  as  to  the  distinctions. 
In  fact  it  is  much  more  difficult  to  establish  an  ordered 
system  in  the  realm  of  smell  than  it  is  has  been  for  any 
of  the  other  senses.  In  discussing  sound  we  have  the 
standard  of  pitch  and  in  describing  visual  sensations 
we  have  the  spectrum.  Odors,  however,  do  not  readily 
fall  into  any  such  continuous  series. 

Man  is  said  to  be  deficient  in  the  acuteness  of  the 


SENSATIONS   AND    THE    SENSE-ORGANS          143 

sense  of  smell,  lagging  far  behind  many  of  the  animals 
with  their  power  to  trail  their  mates  and  their  prey. 
It  can  be  shown  by  microscopic  study  that  in  such 
animals  the  olfactory  nerves  have  notably  extended 
connections  within  the  cerebrum.  Still  it  is  probably 
true  that  the  human  endowment  is  superior  in  some 
respects.  We  can  appreciate  more  varieties  of  so-called 
flavor  (really  odor)  than  cats  and  dogs  can  discriminate. 
We  do  not  seem  to  be  able  to  discover  a  weak  odor  in 
the  presence  of  a  strong  one.  A  dog  tracking  a  rabbit 
through  a  growth  of  sweet  fern  seems  to  do  just  this 
thing.  Yet  it  may  be  that  the  odor  of  the  rabbit  is 
stronger  for  the  dog  than  the  fragrance  of  the  herbage. 

All  sensations  diminish  more  or  less  if  the  exciting 
stimuli  are  long  continued.  This  kind  of  fatigue  is 
marked  in  the  case  of  smell.  Most  odors  cease  to  be 
perceptible  if  the  substance  responsible  remains  with 
us.  People  who  have  been  for  some  time  in  a  room 
which  has  become  close,  or  where  vegetables  are  being 
cooked,  may  not  notice  odors  which  are  very  apparent 
to  a  newcomer.  Our  judgment  as  to  whether  odors 
are  good  or  bad  is  closely  linked  with  our  theories  of 
their  origin.  An  aroma  which  might  be  thought  ap- 
petizing when  proceeding  from  a  cheese  would  de- 
cisively condemn  an  egg. 

Attention  may  be  called  to  the  probable  influence 
of  the  erect  position  upon  the  relative  importance  of 
the  sense-organs.  An  animal  roving  about  with  its 
nose  close  to  the  earth  is  in  a  stratum  of  air  laden  with 
odors.  These  rapidly  diminish  in  variety  and  intensity 
with  elevation.  The  human  nose  is  carried  at  such  a 
height  that  there  are  comparatively  few  sources  of 
stimulation  for  it.  But  the  same  lifting  of  the  head 
above  the  ground  has  somewhat  extended  the  range  of 
hearing  and  vastly  widened  that  of  vision. 

Hearing. — It  has  been  said  that  when  we  taste  any- 
thing we  realize  its  close  contact  with  a  special  part 
of  the  body  surface.  When  we  smell  anything  there  is 


144  HUMAN    PHYSIOLOGY 

also  a  contact  between  some  of  the  material  and  our 
receptors,  but  our  thought  is  of  the  main  mass  of  the 
substance  which  may  be  far  away.  Hearing  and  vision 
are  distinguished  from  the  other  senses  and  superior 
to  them  in  that  it  is  not  matter  but  energy  which  comes 
to  us  from  the  sources  of  stimulation.  The  ear  and  the 
eye  are  called  distance  receptors.  Through  them  our 
universe  becomes  greatly  enlarged. 


FIG.  32. — Semi-diagrammatic  section  through  the  right  ear;  G, 
external  auditory  meatus;  T,  membrana  tympani;P,  tympanic  cavity; 
o,  fenestra  ovalis;  r,  fenestra  rotunda;  B,  semicircular  canal ;S,  cochlea; 
Vt,  scala  vestibuli;  Pt,  scala  tympani;  E,  Eustachian  tube.  (Czermak.) 

We  have  seen  that  the  mechanisms  of  the  internal 
ear  are  useful  to  a  great  degree  in  connection  with  the 
maintenance  of  equilibrium.  In  some  animals  this  is 
probably  their  essential  function.  It  is  not  hard  to  see 
that  organs  sensitive  to  displacements  of  the  body  as 
a  whole  might  also  become  responsive  to  the  slight  and 
repeated  shocks  which  we  call  sound  waves.  In  the 
higher  forms,  including  ourselves,  there  is  a  definite 
division  of  the  labyrinth  to  provide  for  the  two  services. 


SENSATIONS   AND    THE    SENSE-ORGANS          145 

The  part  specialized  for  the  translation  of  sound  waves 
into  nerve-impulses  is  a  spiral  passage  called  the  cochlea. 

Anatomists  distinguish  the  external,  the  middle,  and 
the  internal  ear.  The  external  ear  includes  the  visible 
part  and  the  short  passage  that  is  terminated  by  the 
tympanic  membrane.  This  is  what  is  commonly  called 
the  eardrum,  though  it  is  strictly  the  drumhead.  The 
drum,  regarded  as  a  box,  is  represented  by  an  irregular 
cavity  called  the  middle  ear  or  tympanum  in  the  temporal 
bone.  This  contains  air.  Communication  with  the 
exterior  is  by  way  of  the  Eustachian  tube  which,  on 
either  side,  leads  from  the  tympanum  to  the  upper  part 
of  the  throat  or,  as  one  may  say  with  equal  correctness, 
to  the  back  of  the  nose. 

The  Eustachian  tubes  are  very  narrow;  in  fact  they 
are  usually  closed  by  the  contact  of  their  lining  surfaces. 
At  certain  moments,  as  when  we  swallow  or  blow  the 
nose,  there  is  an  effective  connection.  In  order  that 
the  tympanic  membrane  shall  be  free  to  vibrate  nor- 
mally the  pressures  on  its  two  sides  must  be  equal.  The 
pressure  on  the  outside  is  that  of  the  atmosphere  and 
subject  to  barometric  changes.  The  tympanum  holds 
a  small,  isolated  sample  of  the  atmosphere  and  the 
occasional  opening  of  the  Eustachian  tube  is  neces- 
sary tp  the  standardizing  of  this  sample.  If  the  channel 
of  communication  is  blocked  for  any  length  of  time  an 
inequality  of  pressure  upon  the  two  sides  of  the  membrane 
may  be  expected.  It  will  be  attended  by  the  " stuffy" 
sensation  familiar  in  bad  colds.  When  the  tubes  are 
opened  after  long  obstruction  there  is  a  noticeable  snap 
and  a  marked  relief. 

Many  persons  whose  Eustachian  tubes  do  not  open 
readily  may  notice  the  resulting  discomfort  when  rapidly 
changing  altitude,  for  example,  in  the  ascent  of  a  long 
railroad  grade.  As  the  height  is  gained  there  is  a  lower 
barometric  pressure  and  the  confined  air  in  the  tym- 
panum is  more  dense  than  that  outside.  There  must 
be,  under  these  circumstances,  a  slight  outward  bulging 
10 


146 


HUMAN    PHYSIOLOGY 


of   the   tympanic   membrane.     The   ear   is   functioning 
as  an  aneroid  barometer. 

The  tympanic  membrane  is  adapted  to  vibrate  freely  in 
response  to  a  wide  range  of  vibrations  in  the  air  which 
comes  into  contact  with  it.  It  is  much  more  serviceable 
than  it  could  be  if  it  had  a  strong  tendency  to  take  up 
a  fixed  rate  of  tremor.  In  other  words,  it  has  little 


Spiral 
ganglion. 


Osseous  cod  dear  wall.  Nervus  cochlearis. 

FIG.  33. — Longitudinal  section  of  the  cochlea  of  a  cat.  This  figure 
gives  a  general  view  of  the  cochlea.  The  cochlear  duct  is  met  with  six 
times  in  the  section.  (Sobotta.) 

resonance.  Attached  to  the  membrane  on  its  inner 
surface  is  a  minute  bone  or  ossicle  which  must  vibrate 
with  it.  This  ossicle  transmits  its  motion  to  a  second 
and  this,  in  turn,  to  a  third.  The  three  articulated 
bones  convey  the  vibrations,  without  altering  their 
frequency,  across  the  tympanum  and  to  the  beginning 
of  the  internal  ear. 

The  third  ossicle  is  fitted  into  an  opening  which  is 


SENSATIONS    AND    THE    SENSE-ORGANS          147 

regarded  as  the  entrance  to  the  labyrinth.  The  bone 
is  smaller  than  the  opening  and  the  closure  is  completed 
by  a  pliable  membrane  which  plays  in  and  out  with  the 
excursions  of  the  ossicle.  Beyond  this  point  there  is 
no  more  air.  The  vibrations  are  now  represented  by 
the  pulsations  of  a  clear  fluid,  the  perilymph.  All  the 
numerous  and  intricate  structures  within  the  labyrinth 
may  conceivably  be  affected  by  the  vibrations  introduced 
into  the  perilymph  from  the  ossicles  but,  as  has  been 
said,  the  cochlea  is  the  organ  vitally  concerned  in 
hearing. 

In  a  clean  dry  skull  the  cochlea  is  a  twisted  passage 
suggestive  of  the  interior  of  a  snail  shell,  though  on  a 
small  scale.  It  makes  two  and  a  half  turns  and  its 
diameter  diminishes  toward  the'  blind  end.  Through- 
out its  course  it  receives,  in  the  living  state,  nerve 
fibers  to  a  total  number  of  perhaps  14,000.  What  is 
a  plain  and  undivided  tunnel  in  the  dried  bone  is,  in 
life,  partitioned  into  three  parts  by  two  membranes 
which  stretch  across  it.  The  nerve  fibers  by  means  of 
which  we  hear  originate  in  certain  curious  cell-groups 
associated  with  one  of  these,  the  basilar  membrane  (mem- 
brana  spiralis  of  Fig.  33). 

It  is  supposed  that  the  basilar  membrane  is  shaken 
in  sympathy  with  the  agitation  of  the  perilymph  and 
that  stimulation  of  the  endings  of  the  cochlear  branch 
of  the  auditory  nerve  is  thus  brought  about.  Some 
physiologists  have  been  led  to  believe  that  the  vibra- 
tion rate  of  the  sound  waves  is  reproduced  in  the  rhythm 
of  the  impulses  which  run  from  the  cochlea  to  the 
brain.  The  more  common  view  has  been  that  our  sen- 
sations of  pitch  are  not  due  to  the  rhythm  of  the  im- 
pulses but  to  the  particular  fibers  which  bear  them  at 
different  times.  That  part  of  the  basilar  membrane 
which  is  nearest  to  the  blind  end  of  the  cochlea  is  sup- 
posed to  be  more  sensitive  to  slow  vibrations,  corres- 
ponding to  sounds  of  low  pitch.  Proceeding  thence 
along  the  basilar  membrane  to  the  other  limit  of  the 


148  HUMAN    PHYSIOLOGY 

cochlea  where  it  joins  the  rest  of  the  labyrinth,  we  are 
assumed  to  find  each  succeeding  segment  of  the  mem- 
brane adapted  to  respond  to  a  higher  frequency.  There 
is  some  evidence  from  disease  which  is  favorable  to  this 
view. 

Let  us  trace  what  probably  happens  when  three  pipes 
of  an  organ  sound  a  chord.  Three  vibration  rates  cannot 
strictly  coexist  in  the  air  but  they  are  represented  by  a 
fusion  just  as  they  could  be  on  the  disc  of  the  phono- 
graph. The  tympanic  membrane  moves  back  and 
forth  in  a  fashion  faithful  to  the  details  of  this  com- 
pound motion.  It  behaves  as  the  diaphragm  of  the 
phonograph  or  the  telephone  would  do.  The  ossicles 
transmit  the  peculiar  type  of  movement  to  the  fluid  in 
the  labyrinth.  Pulses  run  through  the  cochlea  and 
the  basilar  membrane  is  shaken.  If  the  common  con- 
ception is  correct  there  are  three  sharply  limited  regions 
of  the  membrane  which  are  thrown  into  energetic 
vibration.  They  correspond  with  the  three  components 
of  the  chord.  The  vibratory  motion  which  was  at  first 
compounded  of  three  elements  is  now  resolved  again  into 
three.  Three  streams  of  impulses  run  simultaneously 
to  the  brain  and  our  sensation  of  harmonious  sound  is 
the  result. 

We  contrast  musical  sounds  with  noises.  The  physical 
difference  lies  in  the  fact  that  a  musical  sound  has  a 
regular  rate  of  vibration  sustained  long  enough  to 
define  its  pitch.  A  noise  has  either  too  many  com- 
ponents to  give  the  impression  of  precise  pitch  or  its 
pitch  is  too  rapidly  shifting.  The  chief  difference  be- 
tween singing  and  speaking  is  that  in  the  first  case  a  suc- 
cession of  definite  pitches  can  be  recognized,  while  in  the 
second  the  vibration  rate  is  changing  every  instant  and 
only  a  rough  judgment  of  average  pitch  can  be  formed. 


CHAPTER  XI 
THE  EYE 

All  kinds  of  organisms  are  affected  by  light.  We  have 
seen  that  its  influence  upon  the  simpler  forms,  es- 
pecially those  not  protected  by  pigment,  is  generally 
destructive.  In  less  intensity  it  acts  as  a  stimulus, 
modifying  the  behavior  of  the  plants  and  animals  upon 
which  it  falls.  Some  cells  retreat  from  it  while  green 
plants  grow  toward  its  sources  to  utilize  its  energy  in 
chemical  syntheses.  Quite  low  in  the  scale  we  notice 
animal  types  provided  with  what  we  call  eye-spots, 
particles  of  peculiar  substance  which  we  believe  to  be 
more  sensitive  to  the  effects  of  light  than  the  other 
protoplasm  of  their  cells. 

There  is  a  great  difference  between  being  influenced 
by  light  and  being  able  to  see.  To  see  we  must  have 
detailed  pictures  formed  upon  the  retinas,  and  we  must 
have  the  requisite  nervous  and  cerebral  connections 
to  make  possible  the  analysis  and  interpretation  of  these 
pictures.  If  ground  glass  were  kept  before  the  eyes  we 
could  estimate  degrees  of  light  and  might  notice  the 
coming  and  going  of  large  shadows.  It  is  probable 
that  the  visual  powers  of  many  organisms,  snails,  for 
example,  are  no  better  than  this.  We  should  be  sub- 
ject to  the  same  limitations  if  the  light  fell  directly 
upon  our  retinas  without  passing  through  the  optical 
systems  which  refract  and  focus  it. 

The  human  eye  is  a  camera.  It  is  a  globe  about  1 
inch  in  diameter  with  a  small  area  projecting  in  front 
as  though  a  second  globe  of  less  diameter  were  imbedded 
in  the  larger  one.  The  region  which  projects  beyond 
the  regular  curvature  of  the  eyeball  is  exquisitely  clear. 

149 


150  HUMAN    PHYSIOLOGY 

It  is  called  the  cornea.  The  rest  of  the  eyeball  is  white 
and  nearly  opaque.  The  eye  is  lodged  in  a  deep  recess 
of  the  skull.  It  has  a  cushion  of  fat  behind  it  and 
six  small  muscles  are  inserted  in  it.  The  optic  nerve 
runs  back  to  the  brain  from  a  point  which  is  not  at  the 
center  of  the  eyeball  but  distinctly  to  the  nasal  side. 

The  eye  is  protected  by  the  lids.  When  these  are 
brought  together  there  is  a  sac  formed  which  extends 
over  about  half  the  eyeball.  It  is  a  mere  slit  when  viewed 
with  reference  to  its  other  dimension  but  it  contains  a 
small  quantity  of  tears.  Glands  under  the  overhanging 


FIG.  34. — The  right  eyeball  with  some  of  its  muscles  in  the  shelter 
of  the  orbit. 

eyebrow  secrete  the  tears  into  the  sac  behind  the  upper 
lid.  The  liquid  spreads  downward  and  inward  over  the 
surface  of  the  eye  to  reach  two  little  openings  near  the 
inner  angle  of  the  lids  whence  a  duct  provides  for  drain- 
age into  the  nose.  The  tears  serve  to  wash  the  eye  and 
preserve  it  from  drying  effects.  The  response  of  the 
glands  when  a  foreign  body  has  struck  the  cornea  is 
reasonable,  but  that  in  emotional  crises  is  hard  to  ac- 
count for. 

The  eyeball  is  described  as  composed  of  three  coats. 
The  outer  one  is  tough  and  dense;  the  cornea  is  a  part 
of  it.  The  middle  coat  is  distinguished  by  the  rich- 


THE    EYE 


151 


ness  of  its  blood-supply;  it  seems  to  be  specially  con- 
cerned with  nutrition.  The  innermost  coat,  reminding 
one  of  the  plate  or  film  in  the  camera,  is  the  retina. 
It  consists  in  part  of  nerve  fibers  which  run  from  every 
region  of  it  to  the  point  of  departure  of  the  optic  nerve. 
This  nerve,  in  leaving  the  eye,  necessarily  perforates  the 
middle  and  the  outer  coats. 

The  middle  coat  adheres  to  the  outer  everywhere  but 


FIG.  35. — A  vertical  section  of  the  right  eye  and  its  lids,  (c)  is  the 
cornea,  (I)  the  crystalline  lens,  its  margins  shielded  by  the  iris,  (s.r.) 
is  the  superior  rectus  muscle,  (i.r.)  the  inferior  rectus.  The  optic  nerve 
(o.ra.)  is  not  cut  by  the  section  but  is  to  be  thought  of  as  lying  back  of 
its  plane,  that  is,  toward  the  nose. 

in  front.  Within  the  circle  of  the  cornea  it  falls  back  and 
forms  the  iris,  pierced  by  a  round  opening,  the  pupil. 
The  iris  is  the  part  which  gives  the  distinctive  color  to 
the  eye.  It  is  provided  with  muscle  fibers  of  the  smooth 
variety  adapted  to  narrow  or  to  widen  the  pupil.  The 
iris  serves  the  same  purpose  as  a  diaphragm  in. a  camera; 
it  limits  the  admission  of  light  when  the  illumination  is 
strong  and  it  has  another  use  which  will  be  pointed 
out  presently.  The  circular  form  would  seem  the  natural 


152  HUMAN    PHYSIOLOGY 

one  for  an  opening  serving  such  purposes  and  it  is  hard 
to  imagine  why  it  should  be  a  vertical  slit  in  the  cat  and 
horizontal  in  the  horse. 

Behind  the  pupil  is  hung  the  dense  but  elastic  body 
which  is  called  the  crystalline  lens.  The  name  is  some- 
what unfortunate  for  it  encourages  the  idea  that  this 
is  the  only  lens  in  the  optic  system.  In  reality  it 
has  a  smaller  share  in  refraction  than  that  borne  by  the 
cornea.  It  is  of  interest  most  of  all  because  it  is  ad- 
justable. It  is  convex  on  both  surfaces  but  less  so  in 
front  than  behind.  The  presence  of  the  lens  separates 
the  interior  of  the  eye  into  a  smaller  cavity  between 
it  and  the  cornea  and  a  larger  one  between  it  and  the 
retina.  Both  these  spaces  are  filled  by  fairly  clear 
material,  the  aqueous  humor  before  and  the  vitreous 
humor  behind  the  lens. 

The  firmness  of  the  eyeball  is  essential  to  its  usefulness, 
for  any  optical  instrument  must  have  a  fixed  form. 
The  eye  resists  deformation  not  so  much  because  of  the 
strength  of  the  outer  coat  as  because  of  the  high  internal 
pressure  which  prevails.  This  is  derived  indirectly  from 
the  pressure  of  the  blood  in  the  arteries  and  the  result 
•may  be  compared  with  the  firmness  developed  in  a 
tire  when  it  is  inflated.  When  the  pressure  of  the  blood 
falls  at  death  the  eye  is  soon  soft  and  sunken. 

Through  the  combined  effect  of  the  cornea  and  the 
crystalline  lens  a  picture  is  made  upon  the  retina.  As 
in  any  camera  the  image  is  upside  down.  It  is  very 
foolish  to  make  much  of  this  fact,  as  people  often  do, 
for  there  is  no  reason  why  we  should  not  become  ac- 
customed to  the  order  of  things  we  have  always  known 
and  grow  to  regard  the  opposite  relation  as  an  inversion. 
One  of  the  first  lessons  learned  by  a  baby  is  to  reach 
in  a  certain  direction  when  an  object  makes  a  certain 
retinal  impression.  If  the  image  is  high  on  the  retina 
he  must  reach  down,  while  if  it  is  low  he  must  reach  up. 
Without  knowing  anything  at  all  about  the  retina  he 
soon  reacts  unerringly. 


THE    EYE  153 

Accommodation. — A  camera  must  be  focused  with 
reference  to  the  distance  of  the  features  to  be  brought 
out  in  the  photograph.  If  it  has  been  used  for  a  land- 
scape and  is  next  to  be  used  for  a  portrait  the  plate  must 
be  set  farther  back  from  the  lens.  The  same  problem 
exists  for  the  eye  but  it  is  not  met  by  changes  of  depth. 
Instead,  the  lens  is  made  to  assume  a  more  pronounced 
curvature,  so  far  as  its  front  surface  is  concerned,  when 
the  attention  is  directed  toward  anything  near  at  hand. 
This  is  accomplished  by  the  contraction  of  smooth 
muscle  distributed  in  the  middle  coat  of  the  eyeball  in 
the  region  surrounding  the  iris.  We  cannot  discuss 
here  the  mechanics  of  the  act.  The  normal  eye  is  de- 
nned as  one  which  forms  clear  images  of  distant  objects 
without  effort.  The  act  of  accommodation  is  for  near 
vision  and  has  a  limit  which  is  easily  discovered,  the 
shortest  distance  at  which  we  can  clearly  see  details. 
When  the  attention  is  shifted  to  something  far  away  the 
adjustment  is  a  passive  one;  we  do  not  speak  of  ac- 
commodation in  this  case  but  of  the  relaxation  of 
accommodation. 

It  can  be  observed  that  when  accommodation  for  near 
vision  is  employed  there  is  a  contraction  of  the  pupil. 
This  is  explained  as  follows.  In  any  lens  that  can  be 
made  the  central  part  is  more  satisfactory  for  the  forma- 
tion of  an  image  than  the  marginal  part.  In  micro- 
scopes, telescopes,  and  cameras  diaphragms  are  used  to 
limit  the  passage  of  rays  to  the  central  portion  of  the 
lenses.  The  more  convex  a  lens  is,  the  more  essential 
this  restriction  becomes.  Therefore,  when  the  crys- 
talline lens  has  been  rounded  for  the  purpose  of  ac- 
commodation, it  is  desirable  to  make  the  pupil  narrower 
than  it  may  be  when  the  lens  is  less  convex.  In  tech- 
nical language,  the  small  opening  is  said  to  "diminish 
spherical  aberration." 

The  extent  to  which  the  lens  can  be  made  to  change  its 
shape  is .  greatest  in  childhood  and  is  progressively 
lessened  with  the  passing  of  the  years.  A  man  with 


154  HUMAN    PHYSIOLOGY 

normal  eyes  who  has  reached  the  age  of  forty-five  usually 
finds  that  he  cannot  focus  small  type.  He  must  begin 
to  use  convex  glasses  for  reading.  It  should  be  pointed 
out  that  when  he  puts  on  his  glasses  he  is  adding  to  the 
total  refracting  power  of  his  optic  system  and  the  prin- 
ciple of  accommodation — extra  convexity — is  still  utilized. 
The  supplementary  lens  is  placed  in  front  of  the  eye  when 
the  lens  inside  can  no  longer  be  rounded  up  to  meet  the 
requirement.  The  loss  of  the  capacity  for  accommodation 
which  comes  with  advancing  age  is  called  presbyopia. 

Defects  of  Vision. — A  person  who  is  near-sighted  has 
an  eyeball  which  is  deeper  than  normal.  The  result  is 
the  same  that  can  be  demonstrated  with  a  camera: 
if  the  distance  between  the  lens  and  the  plate  is  too 
great  for  general  work  there  will  be  sharp  images  of  near 
objects  but  a  blurred  background.  The  act  of  accommo- 
dation makes  a  near-sighted  eye  worse  than  when  at 
rest.  Accordingly,  the  near-sighted  have  little  use  for 
accommodation.  In  old  age  they  may  be  very  proud  of 
the  fact  that  they  can  read  without  glasses. 

Glasses  to  correct  near-sight  must  be  concave.  Their 
effect  is  to  postpone  the  meeting  of  the  rays  which  are 
to  be  focused  until  the  retina  has  been  reached.  Such 
glasses  have  a  function  just  contrary  to  that  of  accommo- 
dation. They  sacrifice  near  vision  in  favor  of  distant. 
Provided  with  them,  the  near-sighted  person  has  normal 
vision  for  distance  and  uses  his  accommodation  power  for 
near  work  as  the  normal  subject  would  do.  Some- 
times a  condition  resembling  near-sight  may  exist  for  a 
time  and  presently  correct  itself.  This  is  due  to  a 
persistent  contraction  of  the  accommodation  muscle 
which  the  victim  cannot  inhibit. 

The  term  far-sight  is  often  used  to  indicate  superior 
visual  power.  But  it  is  better  reserved  to  indicate  an 
abnormality.  The  trouble  here  is  that  the  eyeball  is 
too  shallow.  When  the  nervous  mechanism  is  at  rest 
nothing  is  strictly  in  focus  for  the  far-sighted  eye.  The 
facts  can  be  verified  by  closing  the  bellows  of  a  camera 


THE    EYE 


155 


until  the  plate  is  too  near  the  lens  even  for  the  distant 
landscape.  There  is  this  great  difference  between 
near-sight  and  far-sight :  that  in  the  former  accommodation 
makes  matters  worse  while  in  the  latter  it  establishes 
clear  vision. 

The  sufferer  from  far-sight  gains  a  satisfactory  picture 
of  the  distance  by  using  a  moderate  degree  of  accommo- 


FIG.  36. — (A)  suggests  the  normal  eye,  focusing  parallel  rays,  that 
is,  rays  of  distant  origin.     ' 

(B)  is  a  near-sighted  eye,  outline  dotted;  it  is  too  deep  to  focus  such 
rays  but  adapted  to  certain  rays  from  near  objects. 

(C)  is  a  far-sighted  eye,  outline  dotted,  too  shallow  for  any  focus  until 
the  accommodation  power  is  used. 

dation  effort.  To  read  he  must  redouble  the  strain. 
He  accomplishes  his  purpose,  but  at  a  cost  to  his  nervous 
system  which  is  likely  to  be  evidenced  through  headaches, 
indigestion,  and  other  disorders.  A  convex  glass  sup- 
plies the  extra  refraction  needed  by  the  far-sighted  and 
relieves  him  from  the  necessity  of  providing  it  constantly 
by  his  own  effort.  Comparing  the  near-sighted  and  the 


156  HUMAN    PHYSIOLOGY 

far-sighted  eyes,  and  supposing  that  glasses  are  not  used, 
we  see  that  the  near-sighted  person  does  close  work  with 
a  minimum  and  the  far-sighted  with  a  maximum  of 
strain. 

Astigmatism. — This  is  a  general  term  covering  all 
defects  of  vision  due  to  the  departure  of  any  of  the  re- 
fracting surfaces  from  the  spheric  curvature  which 
such  surfaces  should  have.  The  most  common  type 
for  which  glasses  are  prescribed  has  been  simply  and 
clearly  described  as  "spoon-shaped  cornea.'7  In  the 
language  of  geometry  the  cornea  is  ellipsoidal  instead 
of  spheric.  The  spoon  bowl  which  stands  for  the 
astigmatic  cornea  must  be  thought  of  as  held  with  its 
long  axis  horizontal  in  most  cases.  It  will  then  be 
evident  that  such  a  cornea  curves  more  decidedly  up 
and  down  than  from  left  to  right. 

The  optic  results  of  astigmatism  are  difficult  to  ex- 
plain in  detail.  The  images  can  never  be  wholly  satis- 
factory. When  certain  features  are  sharp  others  will 
be  blurred.  Thus  in  looking  at  a  window  sash  one  who 
has  ordinary  astigmatism  will  not  see  the  upright  and 
the  cross  pieces  with  equal  distinctness.  The  set  par- 
ticularly attended  to  will  be  clearer  than  the  others. 
This  leads  to  a  restless  tendency  to  experiment  with 
the  accommodation  but  whenever  good  definition  for 
one  system  of  lines  has  been  achieved  the  perpendicular 
system  will  have  become  indistinct.  The  strain  of  this 
uneasy  process  may  work  harm  to  the  general  health. 

Glasses  to  correct  astigmatism  must  have  the  astig- 
matic character  themselves  but  in  a  sense  opposite  to 
that  in  the  eyes  for  which  they  are  intended.  If,  as 
usual,  the  excessive  curvature  of  the  cornea  is  up  and 
down  the  lenses  must  curve  less  along  that  meridian 
than  from  left  to  right.  The  best  skill  is  needed  for 
the  designing  and  mounting  of  such  glasses.  If  a  lens 
made  for  this  purpose  is  turned  one-quarter  way  (90°) 
round,  it  will  double  the  defect  which,  in  its  proper 
position,  it  corrects. 


THE    EYE  157 

The  Retina. — It  has  been  said  that  this  coat  of  the 
eye  is  composed  partly  of  the  fibers  which  go  to  make 
the  optic  nerve.  These  fibers  are  not  themselves  directly 
sensitive  to  light.  In  fact  the  place  where  they  con- 
verge to  form  the  nerve  is  a  so-called  blind  spot.  We  do 
not  see  the  images  of  objects  which  are  formed  there. 
The  fact  that  we  are  not  troubled  by  this  deficiency  is 
to  be  explained  chiefly  by  the  circumstance  that  we  are 
so  much  occupied  with  the  central  part  of  the  retinal 
picture  that  we  have  little  appreciation  of  the  outlying 
part.  It  will  be  recalled  that  the  optic  nerve  does  not 
leave  the  center  of  the  retina  but  makes  its  exit  from  a 
point  some  distance  toward  the  nose.  The  central 
spot,  before  mentioned  as  having  the  best  visual  capacity, 
is  called  the  fovea. 

The  student,  when  he  reads  a  description  of  the 
several  layers  of  the  retina,  has  the  strongest  feeling 
that  it  is  all  "  wrong  side  out."  The  cellular  elements 
on  which  the  light  undoubtedly  acts  are  not  arrayed 
upon  the  inner  surface  but  are  on  the  outside,  next  to 
the  middle  coat.  To  reach  them  the  light  must  pass 
through  a  tangle  of  nerve  fibers  and  cells  other  than  the 
true  receptors.  It  must  even  pass  a  network  of  blood- 
vessels. The  layer  which  finally  translates  the  radiant 
energy  into  nerve-impulses  is  that  of  the  rods  and  cones. 
At  the  fovea  the  overlying  matter  is  reduced  and  the 
exposure  of  the  sensitive  units  is  correspondingly  direct. 

The  rods  and  cones  constitute  a  mosaic  pavement  in 
which  the  individual  members  are  placed  with  striking 
regularity.  The  cones  are  rather  more  advanced  and 
elaborate  in  appearance  than  the  rods  and  there  is 
little  doubt  that  they  have  superior  properties.  In  the 
fovea  there  is  a  central  group  of  cones  with  no  rods, 
farther  out  the  cones  are  scattered  among  rods  which 
greatly  outnumber  them,  and  still  farther  from  the 
fovea  no  cones  but  only  rods  are  to  be  found.  This 
distribution  is  associated  with  contrasted  powers  of 
vision  in  these  three  regions. 


158  HUMAN    PHYSIOLOGY 

The  fovea  and  a  zone  extending  some  distance  out- 
side it  have  the  capacity  to  differentiate  all  the  colors. 
Beyond  the  area  with  complete  color  vision  there  is  a 
tract  not  stimulated  characteristically  by  reds  and 
greens  though  distinguishing  yellows  and  blues.  Still 
beyond,  all  color  comparison  is  lost  and  only  light  and 
shade  can  be  recognized.  We  say  that  the  outlying 
region  of  the  retina  is  totally  color-blind.  The  same 
part  is  without  cones,  so  it  has  been  natural  to  infer  that 
the  cones  are  adapted  to  discriminate  color  while  the 
rods  are  affected  in  the  same  way  by  all  varieties  of 
light.  If  this  is  so  their  reactions  can  indicate  nothing 
more  than  the  degree  of  illumination. 

Color-blindness. — What  is  ordinarily  meant  by  this 
expression  is  not  a  complete  inability  to  compare  colors 
but  a  confusion  of  reds  and  greens.  It  will  be  noted 
that  in  any  retina  there  is  a  zone  in  which  this  con- 
fusion exists.  A  person  is  said  to  be  color-blind  when 
the  same  difficulty  extends  even  to  the  fovea.  Since 
red  and  green  are  colors  used  for  railroad  signals  and 
for  the  port  and  starboard  lights  of  vessels  it  has  been 
found  necessary  to  subject  employes  to  careful  tests. 
In  the  trials  the  candidates  are  not  asked  to  name  any 
colors,  as  that  would  be  testing  their  education  rather 
than  their  natural  endowment,  but  they  are  asked 
to  place  in  piles  numerous  skeins  of  yarn,  putting  those 
together  which  have  a  general  similarity. 

It  is  reported  that  this  defect  is  to  be  found  in  one 
man  out  of  about  thirty.  It  is  rare  in  women.  Color- 
blindness is  hereditary  in  families  but,  according  to  an 
odd  principle,  a  man  who  is  color-blind  will  not  have 
color-blind  sons  or  daughters.  His  son's  sons  will  also  be 
free  from  the  defect  and  all  his  granddaughters,  but 
it  may  be  looked  for  in  the  sons  of  his  daughters.  Many 
quaint  stories  are  told  of  the  mistakes  made  by  color- 
blind persons.  A  Dartmouth  student  with  this  handicap 
came  to  Boston  to  attend  the  football  game  with  Harvard. 
At  the  last  moment  his  friends  suggested  that  he  provide 


THE    EYE  159 

himself  with  a  suitable  green  necktie  and  he  hastened 
to  choose  one.  When  he  displayed  it  there  was  a 
strenuous  protest;  it  was  the  Harvard  crimson. 

The  process  in  the  rods  and  cones  must  be  a  photo- 
graphic one.  Some  chemicals  are  present  there  which 
are  changed  in  a  definite  way  by  the  action  of  light. 
The  changes  which  occur  must  furnish  the  immediate 
source  of  the  stimuli  which  start  the  nerve-impulses 
on  their  way  to  the  brain.  One  difference  between  the 
retina  and  a  common  photographic  plate  lies  in  the 
fact  that  in  the  eye  the  taking  of  each  picture  is  fol- 
lowed by  a  marvellously  rapid  recovery  of  a  condition 
which  permits  a  new  set  of  images  to  be  registered.  In 
the  plate  the  outlines  photographed  are  permanent 
even  though  they  may  be  overlaid  by  those  of  a  second 
exposure.  We  may  say  that  in  the  one  case  there  is 
great  resistance  to  fatigue  while  in  the  other  it  is  almost 
immediate.  The  retina,  however,  does  show  a  measure 
of  fatigue  under  strong  stimulation. 

A  retinal  picture,  when  the  eyeball  is  stationary, 
must  consist  of  a  vast  number  of  associated  points. 
In  this  respect  it  resembles  a  half-tone  reproduction. 
In  either  instance  the  points  are  so  numerous  and  so 
close  together  that  no  discontinuous  effect  is  noticed. 
When  we  try,  in  imagination,  to  correlate  these  points 
of  excitation  in  the  retina  with  simultaneous  streams  of 
nerve-impulses  in  thousands  of  fibers  of  the  optic  nerve 
and  with  the  multitudinous  brain-processes  which  re- 
sult from  their  arrival  at  the  centers  we  realize  the 
hopeless  difficulty  of  the  analysis  The  situation  be- 
comes still  more  amazing  when  we  consider  that  we 
can  move  the  eye,  shifting  each  detail  of  the  picture 
from  certain  retinal  cells  to  others,  and  yet  have  the  same 
general  impression  as  before.  By  our  movements  we 
cause  chosen  features  of  the  scene  to  pass  in  succession 
over  the  fovea  while  the  sense  of  the  larger  relationships 
of  all  the  things  we  see  remains  steady  and  reliable. 


160  HUMAN    PHYSIOLOGY 

Binocular  Vision. — How  are  we  served  by  having  two 
eyes  instead  of  one?  In  answering  this  question  it  is 
necessary  to  point  out  that  we  do  not  profit  from  our 
double  equipment  in  the  same  way  that  some  animals 
do.  The  chief  value  of  binocular  vision  in  a  fish  is  that 
stimuli  can  be  received  from  nearly  opposite  directions 
at  the  same  time.  Roughly  speaking,  if  one  eye  is 
looking  east  the  other  is  looking  west.  We  gain  only 
slightly  in  the  width  of  our  visual  field  through  having 
two  eyes.  Our  principal  gain  is  in  what  is  called  stereo- 
scopic vision. 

Stereoscopic  views  are  photographs  taken  in  pairs  from 
points  somewhat  separated.  Such  views  are  not  du- 
plicates. Their  more  distant  features  are  nearly  identical 
but  the  details  of  the  foreground  are  differently  placed 
in  the  two  and  the  more  differently  as  they  are  nearer 
the  camera.  In  the  same  way,  the  picture  on  the  retina 
of  the  right  eye  is  unlike  that  in  the  left  eye  and  most 
markedly  as  regards  things  which  are  close  at  hand.  We 
have  learned  by  experience — without  thinking  about  it 
in  any  analytic  way — that  dissimilarity  of  retinal 
pictures  is  associated  with  nearness  of  objects.  So 
binocular  vision  becomes  a  great  aid  in  forming  judg- 
ments of  distance. 

As  this  is  true  for  distinct  objects  so  it  is  for  various 
parts  of  the  same  object.  Looking  at  a  building  with 
both  eyes  we  have  a  vivid  impression  of  the  projection 
or  recession  of  its  angles.  We  say  that  we  gain  in  this 
way  the  sense  of  solidity.  Stereo-binocular  glasses  are 
made  with  the  object  lenses  farther  apart  than  the  lenses 
which  are  held  to  the  eyes.  As  a  result  the  user  has  not 
only  a  magnified  picture  to  look  at  but  enjoys  the 
advantage  that  would  be  his  if  his  eyes  could  be  moved 
apart  and  made  to  converge  upon  the  scene.  The 
principle  is  that  of  the  surveyor's  triangulation  and  also 
of  the  range  finder. 

Judgments. — In  speaking  of  binocular  vision  we  have 
given  an  idea  of  one  way  in  which  we  estimate  the 


THE    EYE  161 

comparative  distances  of  things  seen.  There  are  other 
ways  in  which  this  is  done.  Even  with  one  eye  we 
have  a  fair  basis  for  drawing  conclusions.  It  may  be 
laid  down  as  a  general  principle  that  motor  acts  are 
attended  with  characteristic  sensations.  This  is  true 
of  the  act  of  accommodation.  When  the  attention  is 
directed  to  a  surface  like  the  page  of  a  book  wMch  is 
only  a  few  inches  away  there  is  a  strong  tension  on  the 
part  of  the  muscle  that  increases  the  curvature  of  the 
lens.  A  feeling  goes  with  the  maintenance  of  this 
tension  and  this  feeling  has,  from  early  life,  been  asso- 
ciated with  looking  at  that  which  is  near  at  hand. 
One  source  of  the  bewilderment  experienced  on  first 
wearing  glasses  is  that  the  accommodation  effort  for  a 
given  distance  is  greater  or  less  than  before.  New 
interpretations  become  necessary. 

When  both  eyes  are  used  there  is  another  muscular 
effort  besides  that  of  accommodation.  This  is  the  strain 
of  convergence,  increasing  as  the  attention  is  directed  to 
things  close  by  and  diminishing  as  we  look  off  to  the 
distance.  It  will  be  evident  that  the  two  eyeballs  must 
be  rotated  considerably  inward  to  bring  the  images  of 
the  same  word  on  the  printed  page  to  both  the  fovese 
at  once.  Failure  to  bring  the  same  images  upon  the 
fovese  results  in  double  vision. 

So  far  we  have  mentioned  the  stereoscopic  principle, 
accommodation  strain,  and  the  sense  of  convergence  as 
contributing  to  our  judgments  of  distance.  Other 
matters  enter  in.  There  is  perspective,  the  fact  made 
familiar  from  experience  that  parallel  lines  retreating 
from  the  eye  seem  to  approach  one  another.  There  are 
effects  of  overlapping  in  the  landscape,  more  distant 
features  projecting  from  behind  those  that  are  not  so  far 
away.  There  are  atmospheric  qualities,  hazy  or  bluish 
appearances  which  we  assume  to  denote  distance.  Then, 
too,  when  we  know  the  size  of  an  object  and  have  an 
unusually  small  image  of  it  formed  upon  the  retina  we 
infer  that  it  is  remote.  So  the  apparent  size  of  the 
11 


162  HUMAN    PHYSIOLOGY 

human  figure  often  helps  us  to  estimate  its  distance  from 
us  and  so  the  distance  of  associated  objects.  These 
devices  can  be  used  in  painting  upon  a  flat  surface 
and  produce  the  desired  effects  in  the  absence  of  stereo- 
scopic properties  and  differential  accommodation. 

Generally  speaking,  we  must  judge  distance  before 
wexcan  judge  size.  It  has  just  been  said  that  the 
magnitude  of  the  image  of  a  familiar  object  helps  us  to 
say  how  far  away  it  is  but  its  size  must  be  certainly 
known  if  this  is  to  be  true.  Bodies  like  rocks,  which 
may  be  of  any  size  within  the  widest  limits,  cannot  be 
judged  as  to  their  actual  bulk  until  their  distance  is 
established.  The  discussions  which  people  indulge  in 
regarding  the  size  of  the  moon  show  how  futile  are  our 
attempts  to  conceive  of  size  when  there  is  no  adequate 
sense  of  the  remoteness  of  the  object  under  observation. 


CHAPTER  XII 
THE  HYGIENE  OF  THE  NERVOUS  SYSTEM 

In  previous  chapters  we  have  given  some  accbunt 
of  the  nervous  system  and  the  sense-organs.  We 
have  seen  that,  from  an  objective  point  of  view,  the 
purpose  of  the  nervous  system  is  to  transmit  impulses 
derived  from  external  stimuli  and  to  apply  them  through 
the  effectors  (muscles  and  glands)  to  secure  useful 
adaptive  reactions.  The  health  or  normality  of  a  sys- 
tem with  such  a  duty  is  clearly  all-important.  In 
this  chapter  we  shall  treat  of  its  conservation  in  the 
briefest  fashion. 

Habit. — It  is  a  property  of  the  brain  to  be  modified 
by  use.  We  say  that  paths  of  easy  transmission  are 
formed  in  it  when  certain  acts  have  been  repeatedly 
performed.  This  is  the  basis  of  skill  and  economy  of 
power.  It  is  nearly  related  to  the  formation  of  habits. 
A  habit  may  be  considered  an  acquired,  personal  re- 
flex, as  we  have  already  suggested.  Things  which  are 
habitual  we  speak  of  as  " second  nature"  and  if  we  stop 
to  consider  what  this  means  we  shall  find  the  idea  to  be 
that  certain  circumstances  lead  regularly  to  certain 
performances. 

The  capacity  to  form  habits  is  of  the  greatest  value. 
As  we  master  one  accomplishment  after  another  we 
are  set  free  from  irksome  attention  to  details  and  can 
attempt  something  new.  A  child  that  is  putting  on  its 
clothes  may  not  be  able  to  think  of  anything  but  the 
task  at  hand;  the  same  is  true  of  the  youth  who  first 
applies  the  razor.  Thanks  to  the  emancipating  virtue 
of  habit-formation,  the  grown  man  dresses  and  shaves 
while  his  mind  is  busy  with  more  interesting  matters. 

163 


164  HUMAN    PHYSIOLOGY 

No  one  would  wish  to  change  those  habits  which  ob- 
viously help  to  support  all  the  useful  activities  of 
life. 

There  are,  on  the  other  hand,  all  manner  of  habits 
which  are  bad  in  the  sense  that  they  detract  from  one's 
efficiency,  injure  health,  or  are  unbecoming.  These 
are  to  be  broken  up  when  recognized  with  all  possible 
energy  and  decision.  Their  exhibition  by  other  people 
is  to  be  viewed  with  a  wholesome  resolution  not  to  fall 
into  such  ways,  though  the  pharisaic  spirit  is  to  be 
avoided. 

One  of  our  wisest  teachers,  the  late  William  James, 
pointed  out  that  there  are  many  habits  which  are  not 
particularly  good  nor  distinctly  bad;  they  are  accepted 
by  our  associates  without  praise  or  blame.  It  was  the 
teaching  of  James  that  such  indifferent  habits  afford  us 
a  most  valuable  sphere  for  mental  training  and  self- 
discipline.  We  can  use  our  will  power  to  change  them 
in  radical  ways,  substituting  one  practice  for  another 
i  by  deliberate  choice.  Such  exercise  is  attended  with  a 
highly  pleasurable  sense  of  being  master  of  oneself  and 
one's  situation.  It  is  easy  to  find  scope  for  the  effort 
in  the  realm  of  our  speech:  to  abandon  a  favorite  phrase 
in  favor  of  another,  to  cast  one's  sentences  in  a  new 
mould. 

This  voluntary  regulation  of  minor  habits  gives  a 
kind  of  control  which  may  be  a  real  asset  in  meeting 
crucial  temptations.  It  is  also  a  means  of  postponing 
old  age.  It  has  been  said  that  the  aged  nervous  system 
is  one  which  records  the  past,  holding  little  promise  of 
future  modification.  Its  "ruts"  are  deep  and  fixed. 
To  change  these  ruts  often  in  early  and  middle  life 
probably  delays  the  coming  of  senile  rigidity.  It  is  to 
some  extent  in  our  power  to  preserve  the  plasticity  of 
the  cerebral  organization  and  it  is  worth  the  cost. 

Inhibition. — The  self-command  that  has  been  recom- 
mended is  largely  a  practice  of  inhibition.  We  have 
said  before  that  this  is  an  eminent  human  capacity. 


THE   HYGIENE    OF  .THE    NERVOUS   SYSTEM       165 

It  is  shown  in  refraining  from  motor  acts  and  quite  as 
truly  in  mental  life.  We  need  two  great  accomplish- 
ments for  intellectual  success,  the  power  of  concen- 
tration and  the  power  of  detachment.  These  seem  to 
be  opposites  but  they  are  really  akin.  To  concentrate 
is  to  banish  thoughts  which  are  irrelevant  to  our  problem 
and  to  detach  is  to  put  aside  the  problem  itself  when  this 
is  desirable.  Great  men  and  women  have  excelled 
in  both  respects;  they  have  been  able  to  narrow  their 
attention  to  the  work  before  them  and,  again,  to  enjoy 
absolute  relaxation  or  to  sleep  when  they  have  thought 
it  opportune. 

Anticipation. — It  is  often  necessary  to  muster  all  our 
resolution  to  exclude  thoughts  of  the  future.  At  other 
times  it  is  the  part  of  wisdom  to  live  much  in  anticipa- 
tion. Under  normal  conditions  one  should  find  interest 
and  happiness  chiefly  in  the  activities  of  the  present. 
Much  is  written  nowadays  of  the  injurious  effect  of 
fear  upon  the  mental  and  physical  health  and  the 
teaching  is  probably  not  carried  too  far.  We  often  see 
splendid  examples  of  detachment  on  the  part  of  those 
who  approach  great  trials,  perhaps  surgical  operations, 
without  dwelling  on  the  ordeals  in  prospect,  taking  an 
unselfish  interest  all  the  while  in  the  ordinary  concerns 
of  the  home.  This  is  heroic  and  at  the  same  time  it  is 
the  best  possible  course  for  the  individual. 

In  temporary  misfortune  anticipation  of  a  better 
day  may  be  beneficent.  At  the  same  time  it  is  some- 
times foolish  to  think  too  much  of  the  future  when 
the  result  is  to  create  discontent  with  present  condi- 
tions. Many  a  homesick  student  makes  himself  more 
miserable  than  is  necessary  by  counting  the  days  to  his 
vacation  and  indulging  in  incessant  plans  for  his  holi- 
days. He  ought  to  find  compensations  in  his  work  and 
his  companionships. 

Emotion. — The  strong  feelings  which,  as  we  say,  at 
times  " possess  us"  are  attended  by  an  outpouring  of 
impulses  from  the  nervous  system  to  various  effectors. 


166  HUMAN    PHYSIOLOGY 

How  extensive  and  potent  is  this  discharge  has  only 
recently  come  to  be  appreciated.  When  a  man  shows 
anger  we  can  recognize  by  the  tension  of  his  muscles 
that  they  are  under  stimulation.  If  he  is  pale  we  may 
conclude  that  his  circulatory  system  is  involved  in  the 
reaction.  His  fast  heart  would  confirm  this  opinion. 
We  may  state  here  what  will  be  more  fully  described 
at  another  time;  that  the  alimentary  canal  and  the 
glands  are  also  played  upon  by  the  impulses  that  issue 
in  moments  of  excitement. 

In  the  light  of  the  facts  emotion  is  seen  to  be  a  very 
real  form  of  exercise.  As  exercise  may  be  wholesome 
or  unwholesome,  reasonable  or  excessive,  so  emotion 
may  count  for  health  or  dispose  to  disease.  People 
who  do  not  have  many  emotional  stirrings  are,  in  a 
sense,  untrained  and  may  be  wanting  in  endurance. 
On  the  other  hand,  intense  and  frequent  emotion  is 
a  source  of  nervous  fatigue.  This  must  now  engage  our 
attention  for  a  little. 

Nervous  Fatigue. — We  have  said  something  about  the 
fatigue  of  the  neuromuscular  mechanism.  It  will  be 
recalled  that  the  motor  end-plates  and  the  muscle 
fibers  themselves  are  subject  to  temporary  loss  of 
capacity  through  use.  The  nerve  cells  may  also  be 
vulnerable;  the  white  matter  is  highly  resistant.  The 
synapses  through  which  effects  are  transmitted  from 
one  unit  to  another  are  quite  susceptible  to  fatigue. 
When  we  have  to  do  with  the  intricate  organization  of 
the  brain  we  cannot  so  easily  analyze  what  we  call  a 
state  of  fatigue.  Fatigue  that  is  severe  and  sharply 
localized  probably  means  in  every  case  a  loss  of  power 
to  act,  but  diffuse  fatigue  in  the  central  nervous 
system  has  another  characteristic;  it  resembles  an 
intoxication  in  which  there  is  much  unprofitable  activity. 

Perhaps  we  may  recognize  two  types  of  nervous  fatigue. 
There  is  the  daily  weariness  which  leads  to  sleep  and  is 
neutralized  in  the  course  of  the  night's  rest.  The 
other  sort  is  more  insidious  and  has  the  perverse  symp- 


THE    HYGIENE    OF   THE    NERVOUS    SYSTEM       167 

toms  of  seeming  stimulation.  It  is  a  state  of  hyper- 
sensitiveness  and  unrest.  The  victim  is  unable  to  relax 
and  aggravates  his  abnormal  condition  because  of  the 
fact.  An  advanced  case  of  nervous  fatigue  may  deserve 
the  name  of  neurasthenia. 

The  neurasthenic  is  too  easily  affected  by  stimuli. 
He  feels  .every  discomfort  more  keenly  than  he  should. 
He  is  distressed  by  the  weather,  his  clothes,  his  chair, 
and  his  bed.  Noises  irritate  him;  so  does  bright  light, 
and  so  do  certain  odors.  His  nervous  system  is  thus  shot 
through  with  many  more  impulses  than  should  properly 
penetrate  it  and  the  responses  are  in  proportion.  There 
are  the  motor  signs  which  we  think  of  as  manifestations 
of  nervousness:  the  continual  shifting  of  position, 
useless  movements  of  hands  and  feet,  tricks  like  twirling 
the  moustache  or  toying  with  the  eyeglasses. 

When  we  have  tangible  evidence  that  the  nervous 
system  is  showering  impulses  upon  the  skeletal  muscles 
it  is  not  hard  to  believe  that  it  is  acting  upon  other 
effectors  in  a  similar  way.  But  the  fact  is  to  be  borne 
in  mind  that  many  of  the  currents  which  run  out  to  the 
viscera  and  the  blood-vessels  are  inhibitory.  So, 
while  a  nervously  tired  person  is  pretty  sure  to  present  a 
picture  of  restlessness,  some  of  his  organs  may  be 
hindered  rather  than  spurred  on  in  their  working. 
This  is  particularly  the  case  with  the  alimentary  canal. 

The  details  of  the  nervous  regulation  of  the  digestive 
tract  can  best  be  taken  up  at  a  later  time.  But  from 
what  has  been  said  it  will  be  plain  that  excessive  activity 
at  the  centers  will  be  likely  to  interfere  seriously  with  the 
execution  of  its  functions.  Nervous  indigestion  is  to 
be  expected.  This  springs  in  part  from  the  suppression 
of  the  secretion  of  the  gastric  juice  and  in  part  from  the 
retarding  of  the  motor  mechanisms  of  the  stomach  and 
intestine.  The  progress  of  food  is  delayed  and  so 
is  its  digestion.  As  a  result  there  is  constipation,  ab- 
normal decomposition  of  the  intestinal  contents,  and 
gas  formation.  More  or  less  poisonous  products  of 


168  HUMAN    PHYSIOLOGY 

putrefaction  may  be  absorbed  into  the  circulation  and 
do  mischief,  a  prominent  consequence  of  such  absorp- 
tion being  an  increased  demoralization  of  the  nervous 
system. 

Other  functions  which  suffer  are  the  action  of  the  heart 
and  the  control  of  the  blood-vessels.  The  last-men- 
tioned disturbance  shows  itself  in  great  sensitiveness  to 
temperature  changes,  the  skin  flushing  or  paling  in 
response  to  comparatively  slight  shifts.  The  perspira- 
tion breaks  out  at  times  in  excessive  volume  and  again 
is  suddenly  checked.  Irregularity  and  instability  of 
reaction  are  to  be  observed  in  every  field.  The  kidneys, 
the  bladder,  and  the  reproductive  organs  misbehave 
and  often  encourage  the  sufferer  in  the  conviction  that 
he  has  grave  local  disease. 

Meanwhile  the  temperamental  signs  are  most  char- 
acteristic. There  is  morbid  absorption  in  self  with  utter 
disregard  of  the  feelings  and  rights  of  others.  Self-pity 
is  a  leading  symptom.  The  feelings  of  the  neurasthenic 
are  always  being  wounded  but  he  has  no  realization  of 
the  injustice  he  inflicts  upon  his  housemates.  He  is 
unreasonable  and  exasperating.  It  is  one  of  the  most 
difficult  of  life's  duties  to  deal  kindly,  firmly,  and  con- 
sistently with  such  people.  They  deserve  sympathy 
but  the  free  expression  of  it  confirms  them  in  their 
pessimism.  The  problem  is  too  large  a  one  to  be  dis- 
cussed in  a  book  like  this. 

Naturally  the  cure  for  nervous  fatigue  must  be  found 
in  rest,  but  the  high  irritability  which  prevails  makes  it 
difficult  to  secure  the  rest  which  is  so  obviously  needed. 
Some  strong  inhibition  must  fall  upon  the  overdriven 
complexes  of  the  nervous  system  before  they  be  restored. 
In  different  cases  this  requisite  inhibition  is  secured 
in  very  different  ways.  Sometimes  hard  and  simple 
manual  work  will  induce  it.  Sometimes  a  deliberate 
withholding  of  sympathy  from  a  nervous  subject,  a  stead- 
fast refusal  to  give  him  the  center  of  the  stage,  has 
worked  wonders.  He  has  silenced  his  own  complaints 


THE    HYGIENE    OF   THE    NERVOUS    SYSTEM       169 

and  mastered  his  own  situation.  In  many  cases  a 
religious  or  semi-religious  ideal,  a  conviction  of  a  great 
and  restful  reality  beyond  oneself,  has  steadied  the 
feverish  system  and  given  it  poise. 

Of  course  the  progress  of  recovery  is  helped  by  many 
conditions  that  may  be  controlled.  The  causes  that  led 
to  the  trouble  are  to  be  removed  so  far  as  they  have 
been  discovered.  They  may  be  physical  disorders, 
calling  for  medical,  surgical,  or  dietetic  treatment. 
The  wearing  of  proper  glasses  may  be  important.  So 
far  as  the  disturbing  factors  have  been  in  the  nature  of 
care,  worry,  domestic  friction,  and  disappointment, 
the  circumstances  must  be  made  easier  where  this  is 
possible  and  fresh  interests  are  to  be  invited.  Life 
needs  to  be  simplified  until  it  does  not  seem  distracting 
but  it  must  not  be  made  stagnant.  Outdoor  exercise 
is  essential. 

Rest  and  Sleep. — We  have  seen  that  extreme  fatigue 
fails  to  bring  automatically  the  rest  which  is  desirable. 
But  ordinarily  in  health  the  inclination  to  rest  after 
activity  is  fairly  trustworthy.  The  question  is  often 
asked  how  far  a  change  is  a  rest  and  how  far  actual 
repose  is  to  be  insisted  on.  It  is  probable  that  some 
strenuous  individuals  have  pushed  the  doctrine  that  a 
change  is  a  rest  farther  than  it  should  be  carried.  It  is 
a  truth  with  limitations. 

The  most  complete  rest  is  in  sleep.  In  this  state  we 
spend  about  one-third  of  our  lives.  Its  nature  is  not 
wholly  clear  but  it  may  be  said  that  to  be  awake  is 
more  wonderful  than  to  be  asleep.  The  evident  fact 
is  that  the  sleeping  brain,  particularly  the  cerebrum, 
is  in  a  condition  of  reduced  activity.  Decerebrate 
animals  still  appear  to  alternate  sleep  and  waking,  thus 
showing  that  the  change  from  one  to  the  other  occurs  in 
some  degree  in  the  brain-stem.  From  our  human 
standpoint  we  think  of  sleep  as  a  suspension  of  con- 
sciousness or  at  least  a  shutting  out  of  impulses  from  the 
receptors  and  a  great  diminution  in  those  going  to  the 


170  HUMAN    PHYSIOLOGY 

muscles.  The  going  and  coming  cannot  entirely  cease; 
breathing  goes  on  and  many  of  the  simpler  reflexes  can 
be  elicited. 

Aside  from  natural  sleep  there  are  two  circumstances 
which  commonly  suspend  consciousness.  One  is  the 
temporary  poisoning  of  the  brain  by  drugs  and  the  other 
the  type  seen  in  fainting  when  the  cerebral  circulation 
has  partially  failed.  Is  our  sleep  more  like  anesthesia 
or  like  fainting?  It  is  likely  that  it  has  points  of  re- 
semblance to  both.  It  is  like  anesthesia  in  that  fatigue 
substances  gathering  in  and  about  the  brain  cells  prob- 
ably dispose  to  it,  but  it  is  like  fainting  also  inasmuch  as 
the  final  relapsing  from  waking  to  sleep  is  believed 
to  depend  on  the  lessening  of  blood  flow  through  the 
brain.  A  person  can  ordinarily  be  waked  quickly  from 
sleep  but  not  from  anesthesia.  The  stimuli  used  to 
overcome  faintness  would  also  serve  to  rouse  a  sleeper. 

The  simplest  way  to  picture  sleep  is  perhaps  to  as- 
sume that  the  central  fact  is  a  high  resistance  affecting 
many  paths.  A  blockade  anywhere  between  the  sense- 
organs  and  the  cerebral  cortex  would  account  for  the 
failure  of  sensation.  A  similar  block  on  the  path  from 
the  motor  areas  to  the  muscles  would  result  in  relaxation 
and  quiescence.  The  interruption  of  association  channels 
would  interfere  with  the  synthetic  processes  of  thought. 
The  dreaming  consciousness,  when  we  realize  it  at  all, 
has  a  character  which  suggests  that  the  sensory  currents 
are  much  impeded  and  we  know  that  the  motor  ex- 
pression is  generally  limited. 

The  first  hour  of  the  night's  sleep  is  one  of  rapidly 
deepening  stupefaction.  This  has  been  proved  by 
measuring  the  intensity  of  stimuli,  such  as  sounds  or 
electric  shocks  required  to  wake  the  sleeper.  After 
the  first  hour  a  shoaling  of  sleep  so-on  begins  and  goes 
on  at  such  a  rate  that  one  would  predict  waking  within 
two  or  three  hours.  Instead  of  this  there  is  a  long 
period  of  light  sleep  from  which  one  is  easily  roused. 
This  is  not  the  same  as  saying  that  one  is  easily  kept 


THE    HYGIENE    OF   THE    NERVOUS    SYSTEM       171 

awake,  for  the  shallow  sleep  toward  morning  is  readily 
resumed  after  an  interruption.  The  nervous  shock 
experienced  when  one  is  awakened  from  the  deep  sleep 
of  midnight  is  vastly  more  severe  than  the  excitement 
created  by  the  alarm  clock  at  6  a.m. 

Certain  writers  on  hygiene,  having  in  mind  the 
maximum  efficiency  of  the  man,  urge  that  sleep  be 
reduced  to  six  or  to  five  hours  a  day.  It  is  hard  to  de- 
cide whether  such  teaching  is  universally  to  be  applied. 
There  would  seem  to  be  a  real  danger  of  overtaxing 
the  nervous  system.  Yet  it  is  probably  true  that  many 
people  waste  time  in  sleep.  This  is  most  apt  to  be  the 
case  with  heavy  eaters  who  are  commonly  heavy  sleepers 
as  well.  Overeating  seems  to  produce  changes  in  the 
composition  and  distribution  of  the  blood  that  favor 
drowsiness.  Consistently  with  this  light  eaters  are 
often  sufferers  from  insomnia. 

When  we  wake  from  sleep  we  may  or  may  not  recall 
that  we  have  been  dreaming.  If  we  have  recollections 
of  the  mental  currents  that  have  run  their  course  while 
we  slept  we  can  usually  say  whether  we  have  been  much 
concerned  with  the  cares  and  interests  of  our  present 
life  or  whether  we  have  been  watching  curious  pictures 
founded  on  scenes  long  past  and,  as  we  supposed,  for- 
gotten. The  best  sleep  is  probably  that  which  seems 
to  have  been  dreamless  as  we  look  back  upon  it  but  if 
there  are  dreams  it  is  not  desirable  that  they  should  be 
of  actual,  present  affairs.  It  is  a  sign  of  nerve-fag  when 
the  thoughts  of  the  night  closely  resemble  those  of  the 
day. 

Alcohol. — While  we  are  dealing  with  the  hygiene  of 
the  nervous  system  some  attention  may  be  given  to  this 
much  debated  subject.  Alcohol  has  other  relations  to 
human  life  than  the  influences  which  it  exerts  upon  the 
brain  but  this  is  by  far  its  most  important  aspect.  It 
is  a  potential  food,  a  relish,  and  sometimes  a  drug,  but 
it  is  not  much  sought  for  these  properties.  It  is  prized 
chiefly  as  a  comforter,  that  is  to  say,  for  its  tempera- 


172  HUMAN    PHYSIOLOGY 

mental  effects.  These  effects  are  apt  to  be  described 
as  stimulation  but  it  is  doubtful  whether  the  word  is 
wisely  used. 

A  group  of  men  who  have  had  some  wine  at  dinner 
are  observed  to  become  talkative  and  animated.  They 
gesticulate  and  laugh  more  than  they  would  ordi- 
narily. They  may  become  decidedly  uproarious.  Is 
this  not  stimulation?  It  has  been  generally  held  to  be, 
but  another  interpretation  can  be  offered.  The  sug- 
gestion has  been  made  that  what  we  are  witnessing  is 
a  withdrawal  of  inhibition,  the  paralysis  of  the  highest 
centers  with  a  consequent  release  of  the  lower  from 
their  regulation.  If  a  train  is  descending  a  grade  there 
are  two  ways  to  account  for  an  increase  of  speed:  more 
power  may  have  been  applied  by  the  engine  or  the  brakes 
may  have  been  taken  off.  The  action  of  alcohol  has 
been  denned  by  most  writers  as  a  taking  off  of  the  cerebral 
"  brakes." 

It  may  be  an  open  question  whether  this  removal  of 
inhibition  is  always  an  evil.  Able  thinkers  have  justified 
the  artifice.  Men  have  resorted  to  it  for  ages  to  secure 
social  ease  and  to  banish  cares.  But  it  does  seem  as 
though  abandoning  oneself  to  convivial  pleasures  should 
be  a  simple  act  of  the  will  rather  than  a  reaction  secured 
by  a  dose  of  narcotic.  We  have  urged  that  the  power  of 
detachment  is  an  accomplishment  to  be  desired  and  culti- 
vated; one  should  not  be  obliged  to  depend  on  alcohol  for 
its  realization.  The  wine  opens  a  royal  road  to  a  goal 
which  it  is  often  well  to  seek,  but  it  is  a  downhill  road 
and  the  return  cannot  be  so  easily  effected  as  the  descent. 
The  man  who  is  really  master  of  himself  can  detach  and 
concentrate  with  equal  success;  the  alcohol  favors  the 
former  but  in  the  same  measure  is  opposed  to  the  latter. 

The  tendency  to  form  a  habit  in  the  use  of  alcohol  is 
not  to  be  made  light  of.  Two  young  men  whose  heredity 
and  principles  seem  to  be  equally  good  may  differ  abso- 
lutely as  to  the  degree  of  safety  with  which  they  can 
drink.  One  may  enjoy  it  from  time  to  time  and  feel 


THE    HYGIENE    OF   THE    NERVOUS    SYSTEM       173 

no  desire  to  increase  the  frequency  or  the  extent  of  his 
indulgence.  The  other  may  prove  to  be  one  of  those 
unfortunates  who  cannot  remain  temperate — one  who 
will  go  on  to  his  own  ruin  and  to  break  the  hearts  of  his 
family.  Total  abstinence  is  a  safe  course  and  it  has 
constantly  a  larger  number  of  distinguished  exemplars. 
Tea,  Coffee,  and  Chocolate. — These  widely  used 
preparations  are  mild  stimulants  in  the  best  senses  of 
the  term.  They  increase  working  capacity  and  at  the 
same  time  may  give  a  sense  of  well-being.  It  is  impossi- 
ble to  prove  that  their  use  is  harmful  for  the  great  major- 
ity of  people.  Many  individuals  have  found  that  these 
beverages  disagree  with  them  and  such  people  are 
usually  sensible  enough  to  abstain.  Some  persons  seem 
unable  to  make  the  sacrifice  where  it  is  desirable  and 
suffer  decidedly  in  consequence.  It  is  a  good  plan  not 
to  resort  to  these  stimulants  when  there  is  no  clear 
occasion  for  their  use.  If  they  are  not  employed 
regularly  they  will  be  doubly  serviceable  in  a  genuine 
emergency. 


CHAPTER  XIII 
THE  ALIMENTARY  CANAL.     DIGESTION 

In  Chapter  II  some  general  statements  were  made  in 
regard  to  the  origin  and  service  of  food.  The  term 
was  used  to  include  things  which  may  enter  into  the 
body  as  constructive  material  or  as  fuels.  By  far 
the  largest  share  of  our  food,  especially  after  the  com- 
pletion of  growth,  is  valuable  purely  as  fuel.  We  have 
now  to  trace  the  preparation  of  food  for  absorption, 
its  distribution  and  storage,  and  the  final  utilization 
of  it  by  the  tissues.  First  of  all  some  attention  must 
be  given  to  the  anatomy  of  the  human  alimentary 
tract. 

The  Alimentary  Canal. — In  all  the  higher  animals 
this  is  a  passage  leading  from  a  mouth  to  a  vent  or  anus. 
The  contents  of  the  canal  are  not  to  be  considered  as 
within  the  body  but  only  in  contact  with  a  part  of  its 
surface.  The  canal  in  man  is  25  or  30  feet  long,  the 
great  length  being  made  possible  by  the  coiling  of  the 
intestine  in  the  abdominal  cavity.  A  long  canal  is  not 
any  more  capacious  than  one  shorter  and  wider,  but  it 
has  more  s}iirffl.nfi  and  this  is  important  since  through  the 
lining  of  the  tract  the  useful  part  of  the  food  is  received 
into  the  blood. 

The  successive  parts  of  the  canal  are  the  mouth,  the 
pharynx,  the  esophagus,  the  stomach,  the  small  intestine, 
and  the  large  intestine  or  colon.  The  large  intestine 
terminates  in  the  rectum  leading  to  the  anal  outlet. 
The  arrangement  of  the  structures  about  the  mouth  is 
sufficiently  familiar.  The  pharynx  is  what  we  often  call 
the  throat,  a  short  section  common  to  the  digestive  and 
the  respiratory  systems.  In  it  the  course  taken  by  the 

174 


THE    ALIMENTARY    CANAL 


175 


B^BBHHB  J 

FIG.  37. — The  human  alimentary  canal  shown  diagrammatically : 
O  is  the  esophagus;  S  is  the  stomach;  S.I.  suggests  the  small  intestine; 
C  is  the  colon;  R  is  the  rectum.  The  connection  between  the  stomach 
and  the  small  intestine  occurs  behind  the  transverse  colon,  which  also 
hides  the  pancreas. 


176 


HUMAN    PHYSIOLOGY 


FIG.  38. — Relations  of  the  mouth  and  nose.  This  is  a  vertical  sec- 
tion through  one  nostril,  and  therefore  slightly  away  from  the  mid-plane 
of  the  head.  The  convoluted  character  of  the  lateral  wall  of  the  nasal 
cavity  is  suggested.  The  connection  between  the  nose  and  the  throat 
will  be  seen  behind  the  soft  palate  (P) ;  L  is  placed  in  the  larynx,  above 
which  is  shown  the  spur  of  the  epiglottis;  O  indicates  the  course  of  the 
esophagus.  It  will  be  noted  that  the  course  taken  by  the  food  crosses 
the  route  of  the  breathing  in  the  pharynx. 


THE    ALIMENTARY    CANAL  177 

air  we  breathe  crosses  that  of  the  food  we  swallow.  The 
larynx,  which  is  ventral  to  the  lower  part  of  the  pharynx 
and  under  the  tongue,  belongs  to  the  respiratory  tract 
exclusively.  The  breath  comes  and  goes  through  the 
larynx  and  the  food  enters  the  esophagus. 

This  is  a  tube  leading  to  the  stomach.  It  runs  down 
behind  the  trachea  or  windpipe  until  the  fork  in  that 
passage  is  reached;  then  the  esophagus  continues  behind 
the  heart  and  pierces  the  diaphragm.  Immediately 
below  this  partition  it  expands  into  the  stomach.  This 
is  a  sac  lying  mainly  to  the  left  of  the  middle  line  and 
higher  up  than  is  popularly  supposed.  It  is  within  the 
ribs  as  viewed  from  the  side  and  could  be  reached  from 
in  front  by  an  incision  just  below  the  end  of  the  breast 
bone.  The  spleen  is  in  the  limited  space  to  the  left  of 
the  stomach  while  the  much  larger  space  between  it 
and  the  right  ribs  is  filled  by  the  great  mass  of  the 
liver. 

The  shape  of  the  stomach  varies  according  to  cir- 
cumstances but  it  may  be  said  to  have  somewhat  the 
form  of  a  pear,  the  small  end  being  directed  downward 
and  to  the  right.  The  small  intestine  takes  its  departure 
from  this  tapering  extremity.  A  line  drawn  from  the 
place  where  the  esophagus  enters  along  the  upper  border 
to  the  place  where  the  intestine  leaves  is  said  to  follow 
the  lesser  curvature.  The  much  longer  line  connecting 
the  same  points  but  following  the  lower  border  defines 
the  greater  curvature.  The  opening  from  the  esophagus 
to  the  cavity  of  the  stomach  is  the  cardia;  that  from  the 
stomach  to  the  intestine  is  the  pylorus. 

The  small  intestine  is  the  longest  division  of  the  canal, 
having  a  course  of  about  20  feet.  It  is  coiled  in  a 
manner  which  defies  description  and  it  ends  by  joining 
the  colon  near  the  right  hip  bone.  Its  first  turn  after 
leaving  the  stomach  is  called  the  duodenum  and  it 
encircles  the  head  of  the  pancreas.  Beyond  the  duo- 
denum the  small  intestine  is  rather  vaguely  divided  into 
two  sections,  the  jejunum  and  the  ileum.  The  words 
12 


178  HUMAN    PHYSIOLOGY 

large  and  small  applied  to  the  intestine  have  reference  to 
the  diameter,  not  the  length. 

The  colon  ascends  on  the  right  side  of  the  body  until 
it  is  overhung  by  the  liver,  then  crosses  to  the  left  just 
below  the  stomach,  and  finally  comes  down  on  the  left 
side  for  a  certain  distance.  Turning  toward  the  back 
it  curves  around  to  the  mid-line,  forming  a  segment 
known  as  the  sigmoid  flexure.  From  this  the  rectum 


FIG.  39. — This  is  an  entirely  schematic  section  across  the  human  body 
in  the  mid-abdominal  region:  S  indicates  the  spine;  K,  the  kidneys;  P 
is  the  peritoneum,  the  lining  of  the  abdominal  wall.  It  is  prolonged 
from  the  back  to  form  the  mesentery  (M),  which  extends  to  and  around 
the  loop  of  intestine  (/) .  The  large  unoccupied  space  shown  does  not 
really  exist,  for  successive  portions  of  the  alimentary  canal  together 
with  other  organs  completely  fill  the  cavity. 

descends  in  front  of  the  lower  bones  of  the  spine  and 
reaches  the  anus.  A  rough  diagram  of  the  colon  re- 
sembles a  question  mark. 

When  the  abdominal  cavity  is  opened  the  first  im- 
pression is  that  the  intestine  lies  loosely  within  like  a 
coil  of  rope.  But  if  a  loop  is  chosen  at  random  and 
lifted  from  its  place  a  clear,  glistening  sheet  of  tissue  is 
found  attached  to  it.  This  is  the  mesentery  and  it  can 
be  followed  to  an  attachment  to  the  dorsal  body  wall. 


THE    ALIMENTARY    CANAL  179 

Thin  as  it  is  it  must  be  thought  of  as  a  double  sheet 
between  the  two  surfaces  of  which  run  the  blood- 
vessels, nerves,  and  lymph-channels  of  the  intestine. 
The  tube  of  the  intestine  itself  should  be  conceived  of 
as  wrapped  round  by  the  mesentery.  One  writer  has 
compared  it  to  the  clothesline  over  which  a  blanket  is 
hung.  Thus  the  continuation  of  the  mesentery  makes 
the  outer  or  serous  coat  of  the  canal. 

At  its  other  border,  where  the  mesentery  reaches  the 
dorsal  boundary  of  abdominal  cavity  its  two  layers 
part  and  spread  to  be  continued  as  the  lining  of  that 
cavity,  the  parietal  peritoneum.  The  shape  of  the 
mesentery  when  entire  is  difficult  to  visualize;  it  must 
be  thought  of  as  following  the  whole  length  of  the  small 
intestine  and  much  of  the  large  and  yet  contracted  to 
join  the  wall  of  the  body  along  a  rather  short  line  of 
insertion.  The  resulting  structure  has  been  compared 
with  a  " ruffle"  or  " flounce."  The  stomach  has  a 
suspending  membrane  which  is  really  a  mesentery  but 
referred  .to  as  the  lesser  omentum.  It  comes  to  the 
lesser  curvature  of  the  stomach  from  the  under  side  of 
the  liver  and  is  continued  over  the  surface  of  that  large 
organ  to  its  attachments  to  the  diaphragm  and  the  back 
of  the  cavity. 

The  serous  coat  of  the  stomach  leaves  the  greater 
curvature  of  the  organ  as  well  as  the  lesser  and  the 
double  sheet  so  formed  hangs  slack  in  front  of  the 
intestine  like  a  short  apron,  the  great  omentum.  This 
is  folded  at  its  lower  limit  and  returns  to  the  transverse 
part  of  the  colon.  The  great  omentum  sometimes  comes 
to  be  a  ponderous  appendage  from  the  accumulation  in 
it  of  adipose  tissue. 

The  Digestive  Glands. — Associated  with  the  ali- 
mentary canal  are  the  organs  which  deliver  into  its 
interior  the  juices  required  to  prepare  the  food  for 
absorption.  These  are  called  glands  but  the  reference 
is  sometimes  to  bulky  organs  like  the  liver  and  some- 
times to  microscopic  features  of  the  lining  of  the  tract. 


180  HUMAN    PHYSIOLOGY 

When  we  take  up  the  minute  organization  of  these  parts 
we  shall  see  that  the  usage  is  reasonable.  Glands  which 
are  placed  distinctly  away  from  the  canal  have  ducts 
leading  to  it  for  the  discharge  of  their  secretions. 

Three  pairs  of  salivary  glands  contribute  saliva  to 
the  mouth.  On  either  side  there  is  a  parotid  gland  below 
and  before  the  ear.  Its  duct  runs  forward  to  empty 
on  the  inside  of  the  cheek  opposite  the  upper  molar 
teeth.  The  submaxillary  gland  is  near  the  angle  of  the 
jaw;  it  has  a  duct  opening  under  the  tongue  close  to  its 
fellow  from  the  other  side.  At  or  near  the  same  point 
comes  in  the  secretion  of  the  small  sublingual  gland  which 
is  under  the  floor  of  the  mouth. 

Glands  of  a  microscopic  order  discharge  by  openings 
that  may  be  spoken  of  as  pores  over  the  entire  lining  of 
the  stomach  and  intestine.  They  secrete  the  gastric 
juice  into  the  stomach  and  the  intestinal  juice  or  succus 
entericus  into  the  intestine.  Their  occurrence  in  the 
lower  divisions  of  the  canal  is  less  abundant  than  higher 
up.  The  pancreas  has  a  chief  duct  uniting  with  the 
small  intestine  just  below  the  pylorus.  At  the  same 
place  the  bile  duct,  bringing  the  secretion  of  the  liver, 
reaches  an  outlet.  The  two  ducts  practically  come  to- 
gether in  the  act  of  entering  the  intestine.  The  spleen 
has  no  duct  and  is  not  strictly  a  gland. 

The  Minute  Structure  of  the  Organs  of  Digestion. — 
The  outer  coat  of  the  stomach  and  intestine  has  already 
been  mentioned,  a  continuation  of  the  mesentery  and 
so  of  the  peritoneum.  This  covers  the  muscular  com- 
ponent of  the  canal  which  is  resolved,  so  far  as  the  small 
intestine  is  concerned,  into  an  outer  layer  in  which  the 
fibers  have  a  longitudinal  direction  and  an  inner  and 
thicker  one  in  which  they  are  transverse  or  circular. 
The  cells  in  these  layers  are  of  the  smooth  type  (Chapter 
IV). 

Inside  the  muscular  coats  there  is  more  or  less  loosely 
woven  tissue  rich  in  blood-vessels  and  nerves.  Still 
within  this  and  next  to  the  hollow  of  the  canal  is  the 


THE    ALIMENTARY    CANAL 


181 


epithelium  or  mucous  membrane  which  calls  for  a  careful 
description.  It  is  a  single  layer  of  cells  which  are  indi- 
vidually prismatic  in  form  with  their  long  dimension 
vertical  to  the  free  surface.  All  that  enters  the  body 
from  the  alimentary  system  must  pass  through  or  be- 
tween these  cells.  The  term  mucous  is  used  because  this 


FIG.  40. — The  principle  of  glandular  structure.  In  the  upper  figure 
a  simple  microscopic  gland  is  supposed  to  be  laid  open  by  a  section  along 
its  vertical  axis.  The  cells  are  seen  to  surround  a  recess  into  which  they 
discharge  their  secretion.  Below,  the  same  structure  is  shown  in  its 
entirety,  and  in  addition  the  encircling  blood-vessels  which  contribute 
to  make  good  the  losses  suffered  by  the  secreting  cells. 

epithelium  is  overlaid  with  a  slimy  film  more  or  less 
protective  in  function,  the  product  of  certain  specialized 
cells. 

The  lining  epithelium  is  not  to  be  thought  of  as  a 
smooth  expanse;  it  is  raised  into  many  prominences  and 
depressed  into  many  pockets.  The  pockets  of  the 
epithelium  are  the  minute  glands  to  which  reference  has 


182  HUMAN    PHYSIOLOGY 

already  been  made.  We  must  now  take  pains  to  make 
clear  the  relations  of  such  glands.  Many  variations  in 
their  shape  are  to  be  found  but  we  may  assume  that  a 
fair  type  is  furnished  by  a  slender  cylindric  pit.  Such 
a  pit  may  be  likened  to  a  well,  the  cells  in  its  walls  being 
quite  suggestive  of  regularly  ordered  masonry.  But 
it  must  be  pointed  out  that  the  cells  close  in  the  bottom 
of  the  gland  where  no  stones  would  usually  be  laid  in 
a  well. 

The  secretion  from  a  gland  is  produced  by  the  cells 
which  bound  the  cavity.  These  cannot  go  on  discharg- 
ing water  and  dissolved  substances  unless  their  losses 
are  made  good.  They  must  receive  supplies  from  the 
lymph  which  lies  at  their  submerged  extremities.  The 
lymph  itself  is  a  limited  source  and  must  be  renewed  by 
the  blood  which  is  led  through  a  network  of  fine  vessels 
in  close  proximity  to  the  secreting  cells.  A  gland  ap- 
pears like  a  filter,  adapted  to  remove  something  from 
the  blood  while  keeping  other  constituents  back.  Secre- 
tion, however,  is  much  more  than  filtration.  The 
material  derived  from  the  blood  is  often  greatly  changed 
during  its  stay  in  the  gland  cells  and  so  we  find  many 
bodies  in  the  product  which  are  not  to  be  found  in  the 
blood. 

It  is  necessary  now  to  show  that  the  custom  of  apply- 
ing the  word  gland  both  to  minute  developments  of  the 
alimentary  mucous  membrane  and  to  massive  organs 
like  the  liver  can  be  justified.  The  fact  is  that  an 
organ  like  the  liver  or  the  pancreas  is  a  vast  aggregate 
of  secreting  recesses  which  individually  are  much  like 
the  simple  glands  of  the  stomach  and  intestine.  The 
branching  ducts  provide  for  the  gathering  of  the  com- 
bined secretions  from  all  these  units.  A  large  gland 
may  be  expected  to  have  a  supporting  capsule  and  par- 
titions of  connective  tissue  subdividing  it  into  lobes  and 
lobules.  It  is  to  be  borne  in  mind  that  many  glands  are 
as  distinctly  under  nervous  control  as  are  the  contractile 
tissues. 


THE    ALIMENTARY    CANAL  183 

Classification  of  Foods. — In  Chapter  II  the  contrast  be- 
tween proteins  and  non- protein  organic  foods  is  briefly 
indicated.  The  former  contain  carbon,. oxygen,  nitrogen, 
hydrogen,  and  sulphur,  sometimes  phosphorus.  The 
latter  are  non-nitrogenous.  They  may  be  grouped  as 
below: 

Carbohydrates,  including  starches  and  sugars. 

Fats  (or  oils). 

Alcohol. 

Starches  are  incompletely  soluble,  of  large  molecule, 
and  tasteless.  They  are  easily  converted  to  sugars 
which  are  freely  soluble,  of  small  molecule,  and  sweet. 
Fats  have  familiar  physical  characters:  low  melting- 
points  and  insolubility  in  water.  They  contain  the  same 
elements  as  carbohydrates:  carbon,  hydrogen,  and  oxy- 
gen (much  more  of  the  first,  much  less  of  the  last). 
Water  and  mineral  salts  are  reckoned  as  inorganic  foods. 
The  Nature  of  the  Digestive  Changes. — Digestion  is 
often  said  to  be  a  preparation  of  food  for  absorption  and 
this  is  correct  if  it  is  understood  in  the  broadest  sense. 
A  narrow  conception  is  to  be  avoided.  First  of  all, 
we  must  not  think  of  digestion  as  mere  solution.  Before 
the  rise  of  organic  chemistry  it  was  scarcely  possible  to 
have  any  other  idea  concerning  it  and  observers  judged 
its  progress  simply  by  the  dissolving  of  food  samples. 
It  is  true  that  solid  food  must  be  dissolved  but  there 
are  other  aspects  of  the  process  to  be  taken  into  account. 
Foods  which  are  in  solution  may  yet  require  to  undergo 
digestion. 

Digestion  is  a  process  of  refining.  It  effects  a  separa- 
tion between  the  valuable  and  the  useless.  But,  again, 
this  is  only  one  feature  of  {he  change.  When  a  food 
already  soluble  is  digested  it  is  said  to  gain  in  diffusi- 
bility.  By  this  we  mean  that  the  power  to  pass  through 
ordinary  membranes,  like  parchment,  is  increased. 
This  may  be  supposed  to  make  absorption  easier  but, 
once  more,  the  gain  in  diffusibility  is  but  one  of  several 
phases  in  the  transformation  which  we  call  digestion. 


184  HUMAN    PHYSIOLOGY 

With  the  advance  of  chemistry  it  was  found  that  the 
digestive  changes  are  always  cleavages,  large  and  com- 
plex molecules  giving  rise  to  new  ones  smaller  and 
more  numerous.  This  reduction  in  the  size  of  molecules 
naturally  favors  diffusion. 

Most  significant  of  all,  digestion  obliterates  many  of 
the  characters  which  differentiate  foods  and  gives  us  at 
last  much  the  same  set  of  products  whatever  the  meal 
may  have  been.  Day  by  day  we  make  different  choices 
but  we  do  not  greatly  alter  the  nature  of  the  contribu- 
tion made  by  the  intestine  to  the  blood.  A  com- 
paratively small  number  of  individual  substances  result 
from  the  serial  cleavages  which  have  occurred.  So 
we  may  say  that  digestion  standardizes  our  food;  it 
prepares  for  the  body  a  few  acceptable  compounds  from 
the  many  strange  and  foreign  ones  which  were  taken 
into  the  stomach. 

The  importance  of  this  standardization  can  be  made 
clear  by  an  illustration.  Take  cane  sugar  as  an  example. 
Here  is  a  food  which  contains  no  waste  matter  and  needs 
no  further  refining.  It  is  soluble  and  diffusible.  Yet 
it  is  not  fit  to  be  introduced  into  the  circulation  and  if 
the  experiment  is  tried  it  will  be  excreted  through  the 
kidneys,  in  other  words,  treated  as  useless  material. 
The  trouble  is  that  it  is  not  a  native  compound.  A 
single  change  quickly  accomplished  in  the  intestine 
transforms  it  into  two  other  kinds  of  sugar  which  the 
body  can  utilize. 

Digestive  Secretions. — The  process  of  digestion  is 
carried  on  under  the  influence  of  the  several  digestive 
juices.  When  one  of  these  is  found  to  have  power  to 
advance  the  digestion  of  a  certain  kind  of  food  we 
naturally  assume  that  an  agent  exists  to  bring  about 
the  observed  effect  and  we  call  the  supposed  agent  an 
enzyme.  Enzymes  are  not  known  in  an  isolated  or 
pure  condition;  their  existence  is  inferred  from  the  be- 
havior of  mixtures  of  a  very  heterogeneous  sort.  We 
say  that  saliva  contains  an  enzyme  capable  of  digesting 


THE    ALIMENTARY    CANAL  185 

starch  and  we  generally  call  the  enzyme  ptyalin  but  we 
are  speaking  of  something  which  is  known  to  us  only  by 
its  action  and  not  by  its  appearance.  Since  the  water, 
salts,  and  mucus  of  the  saliva  do  not  digest  starch  we  are 
warranted  in  saying,  that  something  else  is  there  which 
does  have  this  property. 

Although  we  do  not  know  what  enzymes  are  in  a  strict 
chemical  sense  we  do  know  many  of  their  qualities. 
They  are  destroyed  by  heating  their  solutions  to  tem- 
peratures short  of  boiling.  They  are  restrained  from 
acting  by  cold  but  in  this  case  they  are  not,  as  a  rule, 
prevented  from  resuming  their  action  when  warmed. 
They  are  said  to  be  specific,  the  idea  being  that  one 
enzyme  has  but  one  action.  If  a  digestive  juice  affects 
two  distinct  types  of  food  it  is  considered  to  contain  two 
enzymes.  We  classify  enzymes  according  to  the  com- 
pounds on  which  they  act:  protein-splitting  enzymes 
digest  proteins,  fat-splitting  enzymes  digest  fats,  starch- 
splitting  enzymes  starches,  etc. 

When  we  compare  enzymes  with  other  chemical  agents 
a  most  striking  fact  is  recognized,  namely,  that  enzymes 
are  not  used  up  in  direct  proportion  to  the  work  they 
do.  If  we  are  pouring  hydrochloric  acid  upon  iron 
filings  to  make  hydrogen  gas  we  know  that  we  must 
keep  adding  the  acid  if  we  are  to  continue  to  evolve  the 
hydrogen.  But  if  we  are  turning  starch  to  sugar  by  the 
action  of  saliva  the  amount  of  sugar  formed  depends 
more  on  the  time  than  on  the  quantity  of  the  saliva. 
A  very  small  amount  of  a  digestive  secretion,  containing 
a  much  smaller  amount  of  the  actual  enzyme,  can  act 
continuously  under  favorable  conditions  and  suffer 
only  the  most  gradual  loss  of  virtue  in  the  process. 

The  power  of  an  enzyme  to  carry  on  a  transformation 
without  itself  being  destroyed  might  fail  to  be  evident 
if  the  trial  were  not  carefully  regulated.  If  the  test 
were  made  in  a  flask  the  reaction  would  be  found  to 
lag  more  and  more  until  finally  arrested.  It  might  be 
thought  that  the  enzyme  had  been  exhausted.  But  the 


186  HUMAN    PHYSIOLOGY 

real  cause  of  the  arrest  in  such  cases  is  the  gathering 
of  the  products  of  the  change  in  the  field  of  operations. 
If  the  products  can  be  removed  the  reaction  will  be 
resumed.  We  must  remember  that  the  conditions  in  a 
glass  vessel  must  always  be  much  less  favorable  to  the 
action  of  enzymes  than  those  prevailing  in  the  ali- 
mentary canal.  Nothing  can  escape  from  the  flask  or 
the  test-tube  while  there  is  the  possibility  of  withdrawing 
the  digestive  products  from  the  canal  and  enabling  the 
change  to  proceed. 

There  is  a  certain  temptation  to  speak  of  enzymes  as 
though  they  were  living.  This  is  to  be  guarded  against ; 
they  are  secreted  by  living  cells  but  all  that  they  do  can 
be  explained  without  assuming  that  they  have  life. 
Yeasts,  moulds,  and  bacteria  which  are  simple  living 
things  may  work  upon  solutions  in  which  they  are  grow- 
ing very  much  as  enzymes  in  suitable  variety  would. 
The  result  is  a  fermentation  and  in  all  probability  its  course 
is  determined  largely  by  enzymes  which  the  microor- 
ganisms develop.  But  here  again  the  enzymes  are  not 
themselves  alive  in  the  true  sense  of  the  word.  The  cells 
of  the  growing  culture  are  comparable  with  the  cells  of 
glands,  sources  of  enzymes  which  can  outlast  the  life  of 
their  producers.  There  is  evidence  that  enzymes  may 
be  intracellular,  that  they  may  bear  a  part  in  the  chem- 
ical changes  taking  place  in  protoplasm  itself.  But  most 
of  the  story  of  digestion  can  be  told  without  reference  to 
this  possibility. 

While  the  essential  part  of  digestion  is  chemical  there 
are  physical  accompaniments  that  call  for  recognition. 
The  early  observers  made  much  of  them  and,  indeed, 
it  could  hardly  be  otherwise  for  their  chemical  knowledge 
was  slight.  They  emphasized  the  crushing  and  grinding 
of  the  food  by  the  teeth  and  they  assumed  a  continuation 
of  such  treatment  in  the  stomach  and  beyond.  The 
mechanical  factors  in  digestion  remain  interesting  but 
should  be  regarded  as  preliminary  to  deeper  seated 
changes.  The  salient  fact  in  connection  with  mastica- 


THE    ALIMENTARY    CANAL  187 

tion  or  any  similar  treatment  of  food  is  that  subdivision 
increases  the  surface  exposed  to  the  juices.  This  is 
just  as  true  when  an  oil  is  broken  into  fine  drops — 
emulsified — as  when  we  have  to  do  with  solids. 

Much  that  has  gone  before  can  be  made  clearer  by 
considering  what  kinds  of  food  need  no  digestion.  This 
is  the  case  with  the  simple  sugars,  the  dextrose  of  grapes 
and  honey,  the  levulose  in  many  fruits.  It  is  also  true 
of  the  various  mineral  salts  of  the  diet,  so  far  as  they  are 
destined  to  be  absorbed,  and  of  alcohol.  Any  other  kind 
of  food  can,  theoretically,  be  predigested,  but  the  ad- 
vanced cleavage  products  in  the  case  of  proteins  and 
fats  are  not  appetizing. 


CHAPTER  XIV 
SALIVARY  AND  GASTRIC  DIGESTION 

Before  food  is  placed  in  the  mouth  it  has,  in  many 
cases,  undergone  some  changes  which  anticipate  those 
which  the  digestive  enzymes  are  now  to  carry  forward. 
Many  industrial  and  domestic  processes  have  helped 
to  separate  the  useful  from  the  useless  and  to  subdivide 
the  food.  This  is  true  of  the  milling  of  grain.  The 
ripening  of  fruits  and  vegetables  and  the  analogous 
change  in  meat — these  are  in  the  line  with  digestion  and 
so  far  as  they  have  advanced  they  lighten  the  task  which 
remains  to  be  accomplished. 

Mastication. — In  the  mouth  a  vigorous  mechanical 
treatment  is  administered  to  the  food.  The  lower  jaw 
has  a  complex  movement,  up  and  down,  forward  and 
back,  and  from  side  to  side.  As  a  result  of  this  the  food 
is  not  merely  shaved  and  sliced  (the  particular  work  of 
the  front  teeth  with  their  chisel  form)  but  rubbed  and 
ground  between  the  uneven  surfaces  of  the  molars. 
The  tongue  contributes  much  to  the  process  of  mastica- 
tion, rubbing  portions  of  the  food  against  the  roof  of  the 
mouth  and  constantly  altering  the  position  and  bearing 
of  those  parts  on  which  the  teeth  are  actively  at  work. 

The  Saliva. — While  the  mechanical  operation  goes 
on  the  saliva  flows  from  the  ducts  of  the  glands  in  quan- 
tities of  which  we  have  little  conception.  The  secre- 
tion forms  a  very  large  share  of  what  we  swallow  and 
so  of  the  stomach  contents  after  a  meal.  It  has  been 
estimated  that  3  pints  of  saliva  are  produced  in  a  day. 
The  discharge  is  of  a  reflex  nature  but  is  best  placed 
among  those  reactions  favored  by  certain  states  of 
consciousness  which  have  been  called  psycho-reflexes. 

188 


SALIVARY    AND    GASTRIC    DIGESTION  189 

The  saliva  assists  greatly  in  making  the  food  manage- 
able during  mastication  and  swallowing.  In  some  animals 
it  seems  to  have  no  other  service.  In  others,  including 
man,  it  is  a  real  digestive  juice.  The  fact  has  already 
been  indicated  that  saliva  has  the  power  in  such  cases 
to  convert  starch  to  a  kind  of  sugar. 

This  power  is  referred  to  the  enzyme  ptyalin  or  salivary 
diastase.  If  a  crumb  of  bread  is  held  for  some  time  in 
the  mouth  and  slowly  chewed  a  sweetish  taste  gradually 
develops.  This  is  due  to  the  formation  of  sugar  or  of 
incompletely  digested  compounds,  the  dextrins,  from 
the  starch  of  the  bread.  Someone  has  said  that  we  can 
thus  turn  bread  into  cake.  The  change  made  apparent 
to  the  sense  of  taste  in  this  simple  way  can  be  followed 
almost  as  readily  by  chemical  tests. 

When  starch  is  formed  in  the  leaves  of  plants  it  is  in 
grains  which  give  evidence  of  being  dense  and  brittle. 
Boiling  these  disintegrates  them  and  gives  rise  to  a  paste 
or,  if  the  dilution  is  greater,  to  an  apparent  solution. 
It.  is  fair  to  say  that  boiling  does  not  digest  starch  but 
makes  its  subsequent  digestion  far  easier.  The  surface 
exposure  which  is  so  important  is  almost  infinitely 
multiplied  by  the  destruction  of  the  granules.  Saliva 
makes  comparatively  slow  progress  with  raw  starch  but 
is  wonderfully  efficient  with  that  which  has  been  cooked. 

Starch  gives  an  intense  blue  color  to  a  test  solution 
of  iodin  in  potassium  iodid.  If  we  try  successive  sam- 
ples from  a  mixture  of  warm  starch  and  saliva  we  shall 
find  that  the  color  produced  with  iodin  soon  shifts 
through  a  violet  to  a  red  and  then  fades  until  the  ad- 
dition of  the  digesting  mixture  to  iodin  does  not  darken 
the  color  of  the  reagent  at  all.  A  series  of  tests  showing 
such  results  must  be  taken  to  mean  that  the  starch 
has  rapidly  disappeared.  Another  type  of  test  will 
show  that  sugar  has  taken  its  place.  Most  sugars  are 
said  to  decompose  the  salts  of  copper  while  starch  does 
not.  A  solution  of  copper  hydrate  in  Rochelle  salts 
(Fehling's  solution)  may  be  boiled  with  starch  without 


190  HUMAN    PHYSIOLOGY 

changing  color  but  if  it  is  boiled  with  the  products  of 
salivary  digestion  it  is  bleached  and  deposits  a  red  or 
yellow  sediment  of  the  oxids  of  copper. 

When  a  cow  is  chewing  her  cud  we  may  suppose  that 
there  is  salivary  digestion  in  the  mouth  of  the  ruminant. 
The  average  human  being  is  not  apt  to  hold  food  long 
enough  in  the  mouth  to  allow  much  transformation; 
the  question  is,  rather,  how  long  saliva  can  continue  to 
act  in  the  stomach.  We  can  discuss  this  to  better 
advantage  when  we  have  dealt  with  the  motor  phenomena 
exhibited  by  that  organ. 

Swallowing. — Food  is  transferred  from  the  mouth  to 
the  stomach  by  a  coordinated  series  of  movements  which 
give  an  excellent  example  of  the  reflex  principle  in  an 
elaborate  form.  First,  the  material  is  thrust  back 
into  the  throat  by  the  practical  obliteration  of  the 
mouth  cavity.  Then  the  muscular  bands  in  the  wall  of 
the  pharynx  contract  in  order  from  above  downward, 
squeezing  the  food  into  the  esophagus.  At  this  moment 
breathing  has  to  be  suspended  and  the  passages  closed 
against  the  possible  entrance  of  food.  This  is  ac- 
complished for  the  nasal  connection  by  the  drawing 
back  of  the  soft  palate,  a  mobile  partition  between  the 
mouth  and  the  upper  part  of  the  pharynx.  The  larynx 
is  shielded  by  being  pulled  forward  under  the  root  of 
the  tongue  and,  at  the  same  time,  has  an  additional 
safeguard  through  the  folding  down  upon  it  of  a  leaf- 
like  lid,  the  epiglottis. 

The  first  stages  in  swallowing  are  swiftly  executed. 
When  the  food  is  once  within  the  esophagus  breathing 
can  be  resumed  and  the  bolus  advances  more  slowly. 
It  is  thrust  down  the  esophagus  by  a  travelling  contrac- 
tion, successive  regions  of  the  tube  closing  in  behind  the 
moving  mass  as  one  could  manipulate  a  rubber  tube 
with  the  thumb  and  finger  to  send  a  glass  bead  through 
it.  A  propulsive  movement  of  this  sort  is  an  example  of 
peristalsis.  Careful  analysis  has  shown  that  the  mov- 
ing ring  of  contraction  is  preceded  by  a  zone  of  ex- 


SALIVARY    AND    GASTRIC    DIGESTION 


191 


ceptional  relaxation.  In  about  5  seconds  from  the 
time  the  food  leaves  the  mouth  it  reaches  the  cardiac 
opening  into  the  stomach. 

When  liquid  is  swallowed  the  original  impulse  given  by 
the  reduction  of  the  mouth  cavity  may  send  it  all  the 
way  to  the  stomach  through  the  passive  esophagus. 
It  then  arrives  almost  instantly  but  a  peristaltic  wave 
may  be  expected  to  follow  at  the  usual  slow  rate.  In 
many  cases  the  water  finds  the  cardia  in  a  contracted 
state  and  is  arrested  above  it  until  the  wave  catches  up. 


FIG.  41. — An  exaggerated  representation  of  peristalsis.  I  and  II 
are  successive  views  of  the  same  portion  of  the  alimentary  tube: 
P  is  the  zone  of  contraction  shifting  downward  and  always  preceded 
by  the  zone  of  unusual  relaxation  (N).  Ill  is  an  imaginary  section 
through  II,  showing  the  food  bolus  (6)  slipping  along  in  advance  of  the 
contracting  region,  its  advance  being  facilitated  by  the  relaxation  below. 

When  the  negative,  or  relaxation,  phase  of  the  peristalsis 
comes  along  it  seems  to  involve  the  tissues  around  the 
cardia  and  during  the  consequent  slackening  the  fluid 
enters  the  stomach. 

The  Movements  of  the  Stomach. — A  bird  has  a  thin- 
walled  crop  in  which  food  is  stored  to  be  delivered  slowly 
to  the  gizzard.  This  is  a  highly  muscular  organ  which 
contracts  rhythmically  upon  its  contents  and  has  its 
effect  reinforced  by  the  gravelstones  inside.  Man  does 
not  have  a  crop  and  a  gizzard  but  his  stomach  is  so 


192  HUMAN    PHYSIOLOGY 

differentiated  that  two  regions  suggest  at  least  remotely 
the  two  contrasted  organs  of  the  bird.  Food  is  re- 
ceived at  first  into  the  main  or  fundic  part  of  the  stomach 
which  is  a  pouch  with  rather  thin  walls.  It  has  for  a 
prominent  function  the  immediate  bestowal  of  a  meal. 
An  allied  service  is  to  transmit  food  slowly  to  the  nar- 
rower, right-hand  part  of  the  stomach  leading  to  the 
pylorus.  This  is  the  region  called  the  antrum. 


.XL—. 


FIG.  42. — Above,  the  stomach  is  shown  in  a  distended  but  inactive 
state.  Below,  it  is  creased  by  peristaltic  waves  which  thrust  toward  the 
pylorus. 

The  dotted  line  (d-d)  suggests  the  surface  of  the  diaphragm,  (I)  is  in 
the  space  occupied  by  the  liver,  (sp)  near  the  position  of  the  spleen. 
The  lesser  omentum  and  the  transverse  colon  are  suggested  in  the  upper 
figure. 

The  fundus  relaxes  in  an  accommodating  manner  when 
one  is  eating.  When  the  meal  is  secure,  it  exerts  a 
steady  pressure  upon  it  without  agitating  it  to  any  extent. 
This  sustained  pressure  insures  that,  when  any  of  the 
contents  passes  to  the  intestine,  more  will  promptly  slip 
into  the  antrum.  Two  hours  after  a  meal  the  fundus 


SALIVARY    AND    GASTRIC    DIGESTION  193 

will  be  much  reduced  from  its  maximum  size  but  will 
still  be  pressing  with  little  abatement  of  vigor  upon 
the  material  within.  This  is  an  unobtrusive  action  but 
an  important  one  for  the  success  of  digestion. 

The  antrum  is  the  part  which,  we  have  implied,  has 
certain  affinities  with  a  gizzard.  That  is  to  say,  it  has 
a  relatively  high  degree  of  muscular  activity.  The 
circular  muscle  of  its  walls  is  rather  heavily  developed. 
This  muscle  contracts  at  regular  intervals,  first  near  the 
apparent  beginning  of  the  antrum  to  create  a  deepening 
crease  in  the  contour  of  the  stomach;  then  other  circular 
elements  successively  are  involved  and  the  crease  shifts 
its  position  toward  the  pylorus.  What  is  a  crease 
on  the  outside  is,  of  course,  a  ring  of  constriction  in  the 
interior.  The  tendency  will  be  to  force  small  portions 
of  the  gastric  contents  from  the  stomach  to  the  intestine. 

The  waves  that  march  down  upon  the  pylorus  in  this 
orderly  fashion  usually  find  that  opening  stopped  by  the 
tightened  condition  of  the  muscle  around  it.  The 
result  is  that  the  matter  which  is  being  crowded  upon 
the  pylorus  slips  back  through  the  moving  ring.  The 
peristaltic  movement  is  slow  but  the  reflux  may  be  quite 
brisk.  It  is  probable  that  people  exaggerate  the  energy 
of  the  gastric  contractions.  They  cannot  fairly  be 
said  to  grind  or  crush  the  food;  the  verb  often  used  is 
" churn"  and  this  may  easily  lead  to  a  more  lively 
notion  of  their  effect  than  is  justifiable.  The  safest 
description  is  conveyed  by  the  word  "mix." 

It  will  be  recalled  that  the  smooth  muscle  which 
occurs  in  the  walls  of  the  stomach  is  to  some  extent 
automatic.  In  fact,  all  the  nervous  connections  of 
the  stomach  can  be  broken  and  its  behavior  will  be 
approximately  normal.  Under  such  conditions  the 
movements  observed  are  not  due  to  muscular  properties 
alone  but  also  to  a  kind  of  local  nervous  system  repre- 
sented by  cells  and  networks  of  fibers  in  the  organ 
itself.  The  nerves  which  influence  the  stomach  from 
without  may  either  increase  or  diminish  its  activity 

13 


194  HUMAN    PHYSIOLOGY 

from  the  prevailing  average.  Inhibition  is  seen  more 
commonly  than  augmentation. 

It  is  most  interesting  to  discover  that  the  tonic  con- 
traction of  the  fundus  and  the  peristalsis  of  the  antrum 
both  may  be  interfered  with  as  a  result  of  psychic 
factors.  These  movements  have  often  been  studied 
by  means  of  the  X-rays.  (This  is  accomplished  by 
mixing  a  substance  with  the  food  which  will  intercept 
the  rays  and  so  image  the  contents  of  the  stomach  as 
a  silhouette  on  a  suitable  screen.)  It  has  been  proved 
by  repeated  observations  that  the  stomach  of  an  angry 
or  frightened  animal  ceases  to  work  and  remains  in  a  re- 
laxed condition  until  the  animal  is  pacified.  The  hygienic 
suggestion  is  obvious.  It  will  be  emphasized  later. 

Hunger. — When  a  person  has  been  for  some  time 
without  food  well-marked  pangs  may  be  experienced. 
These  are  referred  to  the  stomach  or  to  the  lower  end 
of  the  esophagus  and  they  come  and  go  at  intervals. 
These  gnawings  may  be  considered  to  be  sensations  of 
hunger  as  distinguished  from  appetite.  Appetite  is 
a  matter  of  association,  involving  memories  and  antici- 
pations; hunger  is  a  simple  physical  symptom  piercing 
to  consciousness  and  known  to  infants  and  to  the  lower 
animals  as  well  as  to  adults.  Ingenious  experiments 
have  shown  that  the  pangs  of  hunger  are  associated 
with  strong  contractions  of  the  fundus  of  the  stomach. 
This  may  well  account  for  the  fact  that  these  sensations 
are  often  accompanied  by  sounds  due  to  the  shifting 
about  of  gas  in  the  empty  organ. 

It  is  probable  that  the  hunger  pangs  are  much  more 
marked  in  some  individuals  than  in  others.  In  the 
course  of  a  long  fast  they  become  weaker  and  may 
cease  altogether  in  two  or  three  days.  So  it  happens 
that  absolute  fasting  is  described  as  not  distressing 
while  insufficient  feeding  is  certainly  productive  of 
real  suffering.  The  most  harrowing  stories  of  misery 
endured  by  Arctic  and  other  explorers  have  had  reference 
rather  to  short  rations  than  to  downright  starvation. 


SALIVARY    AND    GASTRIC    DIGESTION  195 

The  Sphincters. — Where  a  circular  zone  of  muscle  is 
habitually  contracted  to  close  a  passage  we  say  that  a 
sphincter  exists.  The  idea  is  not  so  much  of  a  definite 
structure  as  of  a  special  manifestation  of  irritability. 
We  say  that  sphincters  are  found  at  the  cardia  and  the 
pylorus,  meaning  that  closure  at  these  places  is  the  rule 
rather  than  the  exception.  The  conditions  which  de- 
termine whether  there  shall  be  contraction  or  relaxa- 
tion at  these  localities  have  been  much  studied.  Much 
is  found  to  depend  upon  the  degree  of  acidity  of  the 
stomach  contents. 

The  cardiac  sphincter  is  not  so  tightly  contracted  just 
after  a  meal  as  it  is  a  little  later.  I  The  rustic  diner  usually 
concludes  his  repast  by  freeing  his  stomach  of  the  air 
he  has  swallowed  and  has  no  difficulty  in  doing  so\  The 
X-ray  has  shown  that  for  a  short  time  after  a  meal 
the  food  eaten  by  a  cat  slips  out  again  and  again  into  the 
lower  part  of  the  esophagus  to  be  returned  methodically 
by  peristaltic  waves.  Very  soon  this  escape  becomes 
impossible.  The  secretion  of  the  gastric  glands  is 
strongly  acid  and  the  effect  of  acid  acting  just  below  the 
sphincter  is  to  increase  its  tone.  This  is  consistent 
with  the  fact  that  a  swallow  of  alkali,  such  as  magnesia 
or  bicarbonate  of  soda,  favors  the  relaxation  of  the 
sphincter  and  the  escape  of  gas.  The  central  nervous 
system  appears  to  override  the  local  influence  of  the 
acid  at  the  time  of  vomiting  when  the  opening  of  the 
sphincter  occurs  often  in  spite  of  high  acidity. 

In  general,  it  may  be  said  of  the  pylorus  that  it  reacts 
in  the  same  way  as  the  cardiac  opening  of  the  stomach. 
A  period  of  strong  closure  follows  each  passage  of  acid 
material  to  the  duodenum.  The  mechanism  thus  pro- 
vides that  after  each  transfer  of  food  from  the  stomach 
to  the  intestine  there  shall  be  a  period  of  constriction. 
One  cannot  picture  a  simpler  or  better  device  to  guard 
against  overdistention  of  the  intestine;  time  is  allowed 
for  each  succeeding  portion  to  make  its  way  onward 
before  any  more  follows  it. 


196  HUMAN    PHYSIOLOGY 

Vomiting. — When  the  stomach  is  to  be  emptied 
through  the  cardia  instead  of  in  the  normal  way  the 
antrum  is  said  to  contract  while  the  opening  to  the 
esophagus  yawns.  The  diaphragm  bears  strongly  down 
and  the  abdominal  muscles  spasmodically  grip  the 
viscera.  The  sudden  pressure  projects  more  or  less  of 
the  gastric  contents  up  the  esophagus.  The  respiratory 
passages  are  protected  as  in  swallowing  by  the  tucking 
of  the  larynx  under  the  tongue  and  the  swinging  of  the 
soft  palate  to  the  back  of  the  pharynx.  The  chest  is 
alternately  expanded  and  compressed,  the  expansions 
enlarging  the  esophagus  for  the  stomach  to  fill  and  the 
compressions  hurrying  the  accumulation  to  the  exterior. 
This  reflex  is  one  in  which  the  breathing  muscles  are  far 
more  actively  employed  than  those  of  the  stomach. 
Its  value  in  removing  substances  that  might  prove 
poisonous  is  perfectly  clear  but  it  occurs  under  many 
circumstances  when  it  seems  to  serve  no  useful  purpose. 

Salivary  Digestion  in  the  Stomach. — The  digestion 
of  starch  by  the  saliva  is  limited  by  the  development  of 
acidity  in  the  stomach.  The  enzyme  of  the  ^saliva 
is  destroyed  by  acid  in  a  minute  percentage.  The  es- 
sential question  is  how  long  a  time  may  pass  before  the 
acidity  is  developed  throughout  the  mass  of  the  food. 
This  mass,  we  have  seen,  lies  nearly  motionless  in  the 
fundus.  The  incoming  gastric  juice  works  its  way 
from  the  walls  of  the  stomach  gradually  toward  the 
center.  Minute  by  minute  the  sphere  dominated  by 
the  acid  will  be  larger  and  the  core  in  which  salivary  di- 
gestion can  go  on  will  be  more  restricted.  Yet  it  is  likely 
that  for  half  a  hour  or  more  after  an  average  meal  some 
space  remains  in  which  starch  digestion  is  in  progress. 

The  transformation  of  starch  by  the  saliva  is  now 
believed  to  constitute  an  important  part  of  the  digestive 
process,  something  which  was  not  held  to  be  true  a  few 
years  ago.  Tests  which  have  been  made  upon  pre- 
scribed portions  of  food  swallowed  by  individuals  and 
recovered  after  various  intervals  have  shown  that  a 


SALIVARY    AND    GASTRIC    DIGESTION  197 

rather  large  share  of  well-cooked  starch  is  turned  to 
dextrins  and  sugar  in  the  stomach.  There  is  no  doubt 
that  the  fullest  measure  of  salivary  digestion  is  to  be 
desired  since  it  lessens  the  work  which  remains  to  be 
done  in  the  intestine  and  it  makes  the  proteins  more 
accessible  to  the  action  of  the  gastric  juice.  We  must 
now  pass  to  a  discussion  of  the  formation  and  characters 
of  this  secretion. 

Gastric  Juice. — This  is  a  clear,  free-flowing  liquid 
which  is  furnished  at  and  after  each  meal  by  the  simple 
glands  in  the  lining  of  the  stomach.  The  major  part, 
secreted  in  the  fundus,  is  strongly  acid;  the  smaller 
contribution  of  the  antrum  is  neutral  or  alkaline.  The 
appearance  of  the  drops  of  the.  gastric  juice  upon  the 
mucous  membrane  of  the  stomach  is  described  as  like 
that  of  profuse  perspiration  on  the  skin.  The  acid  is 
hydrochloric;  it  must  have  come  from  the  chlorids  of 
the  blood.  It  amounts  to  as  much  as  0.2  per  cent,  in 
the  human  subject  and  may  be  somewhat  more. 

The  activity  of  the  gastric  glands  depends  much  upon 
the  higher  centers.  The  flow  seems  to  begin  in  advance 
of  the  actual  arrival  of  food  in  the  stomach  and  to  be 
proportional  to  the  pleasure  afforded  by  the  meal. 
Anger  and  anxiety,  the  same  psychic  disturbances  which 
may  abolish  the  movements  of  the  stomach,  may  also 
prevent  the  production  of  the  gastric  juice.  We  are 
not  likely  to  overestimate  the  advantages  of  attractive 
food  and  congenial  society  about  the  table. 

Skillful  experiments  upon  dogs  have  shown  that  it  is 
only  necessary  for  the  animals  to  taste  and  chew  food 
to  have  a  lively  secretion  of  juice.  The  morsels  swal- 
lowed may  be  diverted  to  come  out  through  an  opening 
in  the  neck  and  thus  never  reach  the  stomach,  yet  the 
gastric  glands  are  active  as  long  as  the  dogs  are  enter- 
tained. Human  beings  who  have  had  artificial  openings 
made  into  the  stomach  on  account  of  permanent  ob- 
struction of  the  esophagus  still  find  it  beneficial  to  take 
samples  of  their  food  into  the  mouth  and  to  dwell  upon 


198  HUMAN    PHYSIOLOGY 

their  agreeable  qualities.  The  paths  of  the  impulses 
which  go  from  the  brain  to  the  stomach  at  such  times  to 
excite  the  secreting  cells  are  in  the  vagus  nerves. 

Some  foods  which  are  of  the  highest  nutritive  value — 
eggs,  for  example — may  be  introduced  into  the  stomach 
of  a  dog  and  no  secretion  will  start  if  the  dog  has  not  had 
the  opportunity  to  enjoy  the  meal.  This  by  itself 
might  lead  to  the  belief  that  no  gastric  juice  is  produced 
except  under  the  stimulus  of  pleasure  but  this  is  not 
actually  the  case.  Some  foods  produce  the  reaction  by 
their  effect  on  the  lining  of  the  stomach.  This  has  been 
shown  to  be  true  of  meats.  It  is  claimed  that  the 
dextrins  formed  in  salivary  digestion  have  the  same 
desirable  action. 

Compounds  which  have  the  power  to  excite  the  gastric 
glands  directly  are  known  as  seer etag agues.  We  are 
quite  sure  that  meat  contains  bodies  deserving  this  name 
but  there  is  much  disagreement  concerning  the  right  of 
other  things,  such  as  alcohol  and  condiments,  to  the 
title.  If  the  dextrins  are  secretagogues  the  fact  is  of 
interest  because  it  indicates  a  connecting  link  between 
the  salivary  and  the  gastric  process.  A  successful  sali- 
vary digestion  would  prepare  the  way  for  an  adequate 
gastric  secretion.  Foods,  like  eggs  which  contain  no 
secretagogues  when  undigested,  are  thought  to  yield 
something  of  the  kind  when  their  constituents  have 
undergone  some  cleavage  in  the  stomach. 

Summarizing  the  causes  of  gastric  secretion  we  may 
say  that  it  is  induced  (1)  by  pleasurable  feelings  or  (2) 
by  certain  stimulating  substances  in  the  food.  What- 
ever the  means  by  which  it  is  at  first  excited  it  is  main- 
tained in  the  later  hours  of  the  gastric  digestion  by 
secretagogues  arising  as  by-products  of  the  process 
itself.  The  formation  of  such  compounds  will  cease 
only  when  the  stomach  is  empty,  so  the  continuance  of 
the  flow  until  that  time  is  assured. 

Functions  of  the  Acid. — The  hydrochloric  acid  of  the 
gastric  juice  is  necessary  to  the  type  of  digestion  which 


SALIVARY   AND    GASTRIC    DIGESTION  199 

that  juice  effects.  We  have  seen  that  it  has  much  to 
do  with  the  regulation  of  the  two  sphincters.  We 
shall  find  later  that  the  awakening  of  the  pancreas  to 
activity  at  a  proper  time  depends  to  a  great  degree 
upon  the  amount  of  this  acid.  Still  another  function  is 
expressed  when  we  say  that  the  acid  makes  gastric 
juice  antiseptic.  The  microorganisms  which  may  have 
been  swallowed  in  the  food  are  not  all  killed  by  the 
secretion  but  many  of  them  arc  while  others  are  prob- 
ably weakened.  It  is  likely  that  many  germs  which 
might  cause  disease  are  destroyed  in  the  stomach. 
The  suggestion  has  been  made  that  infected  drinking 
water  is  more  dangerous  than  similarly  infected  food 
because  water  alone  induces  only  a  slight  flow  of  gastric 
juice. 

Rennin. — Gastric  juice  has  one  curious  property  which 
has  been  familiar  from  ancient  times.  This  is  the  power 
to  curdle  milk.  Extracts  made  from  the  lining  of  the 
calf's  stomach  have  been  used  for  ages  in  making  cheese. 
Under  the  influence  of  such  preparations  the  bulk  of  the 
protein  in  milk,  the  casein,  is  changed  from  a  soluble  to 
an  insoluble  form.  This  is  opposed  to  our  general 
conception  of  digestion  as  a  process  in  which  the  solubility 
of  the  products  is  steadily  increased.  The  curd  has 
later  to  be  resolved  like  any  solid  food.  No  satisfactory 
reason  for  the  existence  of  the  coagulating  agent  can 
be  offered.  An  enzyme  is  assumed  to  be  responsible 
and  it  is  called  rennin. 

Peptic  Digestion. — The  chief  action  of  the  gastric 
juice  is  upon  proteins.  These  compounds  have  been 
briefly  characterized  in  Chapter  II.  They  are  present 
in  most  foods  but  most  abundantly  in  meats,  eggs, 
milk,  legumes,  and  cereals.  In  the  last  two  classes  of 
staples  they  are  associated  with  starch.  The  greater 
part  of  the  proteins  we  eat  are  in  a  solidified  or  coagulated 
form  as  a  result  of  heating.  The  effect  is  familiarly 
observed  in  the  boiling  of  eggs.  Whether  coagulated  or 
not  they  require  extensive  digestion  and  this  is  begun 


200  HUMAN    PHYSIOLOGY 

by  the  gastric  juice.  The  enzyme  is  generally  known 
as  pepsin.  An  acid  medium  is  essential  to  its  action. 

Solid  proteins,  like  boiled  white  of  egg,  first  swell  in 
gastric  juice  and  then  dissolve.  If  the  sample  is  origi- 
nally liquid,  like  raw  white  of  egg,  the  change  is  just  as 
definite  though  it  cannot  be  followed  by  the  eye.  Vari- 
ous stages  in  the  digestion  have  been  described  by  in- 
vestigators but  they  need  not  concern  us.  The  products 
which  we  may  expect  to  find  in  the  place  of  the  original 
protein  after  the  usual  period  in  the  stomach  are  highly 
soluble,-  fairly  diffusible,  and  bitter.  They  are  spoken 
of  as  peptones.  They  are  susceptible  of  further  cleavage 
but  this  is  more  likely  to  be  accomplished  in  the  in- 
testine than  in  the  stomach. 

Fat  Digestion  in  the  Stomach. — There  is  little  change 
in  a  pure  fat,  like  butter  or  lard,  during  the  time  that  it 
remains  in  the  stomach.  Emulsified  fats,  like  cream, 
are  acted  upon  to  a  limited  extent,  the  products  being 
glycerin  and  fatty  acids.  The  enzyme  to  which  this 
effect  is  credited  is  called  gastric  lipase.  It  is  certainly 
much  less  important  than  the  lipase  from  the  pancreas 
which  is  to  reach  the  food  in  the  small  intestine.  What 
we  call  the  fat  of  meat  is  really  something  more  than 
fat  in  the  strict  sense.  Fat  is  there  in  abundance  but 
it  is  retained  by  envelopes  and  fibers  which  are  of  a 
protein  character.  The  supporting  material  of  the 
adipose  tissue  is  subject  to  peptic  digestion  and  when  this 
is  accomplished  the  true  fat  separates  as  an  oil  which  the 
gastric  juice  scarcely  attacks. 

Fermentation  in  the  Stomach. — Emphasis  has  been 
placed  on  the  antiseptic  virtue  of  the  gastric  juice.  The 
normal  acidity  prevailing  after  a  meal  is  unfavorable 
to  most  types  of  bacterial  activity,  but  it  is  natural  to 
expect  that  if  any  kinds  of  organisms  are  permitted  to 
multiply  they  will  be  those  which  produce  acid  by  their 
own  life-processes.  There  is  usually  some  bacterial 
decomposition  going  on  in  the  stomach,  affecting  prin- 
cipally the  sugars,  and  yielding  lactic  acid  as  a  chief 


SALIVARY   AND    GASTRIC    DIGESTION  201 

product.  This  is  very  like  what  takes  place  in  the  sour- 
ing of  milk,  and,  indeed  milk  may  be  soured  in  the 
stomach  within  a  very  short  interval.  Lactic  acid  is 
not  harmful  unless  in  uncommon  amounts;  many  people 
who  make  use  of  buttermilk  or  other  beverages  contain- 
ing it  believe  that  it  is  beneficial. 

Summary. — The  stomach  serves,  first  of  all,  as  a  place 
for  storage  and  to  maintain  a  gradual  delivery  to  the 
intestine.  The  antrum  has  an  action  adapted  to  pro- 
mote the  mechanical  reduction  of  the  food.  Salivary 
digestion  of  starch  proceeds  in  the  stomach  until  the 
acidity  is  everywhere  established.  True  gastric  diges- 
tion affects  proteins  chiefly  and  is  incomplete.  There 
is  some  digestion  of  well-emulsified  fat.  The  mixture 
passing  the  pylorus  two  or  three  hours  after  a  mixed 
meal  may  be  expected  to  contain  peptones  derived  from 
the  proteins,  dextrins  and  sugar  from  the  starch,  traces 
of  glycerin  and  fatty  acids  from  fats,  portions  of  all  the 
foods  undigested,  hydrochloric  acid  from  the  gastric 
juice,  and  lactic  acid  produced  by  fermentation.  Except 
for  particularly  resistant  particles  it  will  be  smooth  and 
creamy. 


CHAPTER  XV 
INTESTINAL  DIGESTION.    ABSORPTION 

The  three  secretions  which  enter  the  intestine  are 
all  alkaline.  It  does  not  follow  that  the  contents  will 
be  alkaline  throughout  for  there  are  sources  of  acidity 
to  be  reckoned  with.  Three  of  these  are  notable:  there 
is  the  acid  received  from  the  stomach,  more  which  is 
formed  by  the  continued  fermentation  of  sugars  in  the 
intestine,  and  still  more  formed  in  the  normal  diges- 
tion of  fats.  All  these  acids  may  react  with  the  car- 
bonate of  soda  present  in  the  juices  and  there  may  be 
an  excess  of  acid  in  some  sections  of  the  canal.  But  the 
average  condition  is  not  far  from  neutral. 

Immediately  below  the  stomach  the  acid  material 
meets  the  inflowing  bile  and  the  pancreatic  juice  together 
with  some  of  the  succus  entericus.  When  acid  reacts 
with  carbonate  of  soda  an  evolution  of  carbon  dioxid 
is  to  be  expected  and  if  this  is  too  abundant  to  be  held 
in  solution  there  will  be  an  actual  effervescence.  It  has 
been  thought  that  this  may  be  a  useful  factor,  lightening 
the  texture  of  the  food  particles  as  the  gas  bubbles  in 
the  dough  lighten  the  loaf. 

Movements  of  the  Small  Intestines. — Four  or  five 
hours  after  a  meal  the  stomach  will  probably  be  thrust- 
ing out  the  latest  portions  of  its  contents  to  the  duo- 
denum. At  about  the  same  time  the  foremost  frac- 
tions may  be  entering  the  large  intestine.  It  is  then 
that  food  may  be  undergoing  digestion  in  every  loop 
of  the  small  intestine  while  absorption  will  be  at  its 
height.  The  average  rate  of  progress  in  the  intestine 
is  evidently  not  more  than  an  inch  in  a  minute.  This 
is  vastly  slower  than  the  rate  of  travel  in  the  esophagus 
which  is  about  2  inches  in  a  second.  The  mechanical 

202 


INTESTINAL   DIGESTION  203 

principle  is,  nevertheless,  the  same  in  the  two  places. 
The  low  rate  in  the  small  intestine  is  due  partly  to  slower 
muscular  action  but  also  to  the  fact  that  it  is  a  dis- 
continuous movement ;  a  given  collection  of  food  is  often 
at  rest  for  a  while. 

A  peristaltic  wave  in  the  small  intestine  as  in  the 
esophagus  seems  to  have  two  phases,  a  region  of  less- 
ened tone  running  ahead  of  a  ring  of  contraction. 
The  nervous  mechanism  of  the  intestinal  wall  is  capable 
of  actuating  peristalsis  without  the  aid  of  the  central 
nervous  system.  It  may  be  said  of  the  small  intestine 
as  of  the  stomach  that  inhibition  is  more  commonly 
exercised  by  the  central  gray  matter  than  augmentation, 
although  both  are  possible.  Where  there  is  an  accumu- 
lation of  food  and  secretions  a  peristaltic  wave  is  likely 
to  make  its  appearance  and  to  push  the  contents  along 
for  some  distance,  but  soon  the  energy  of  the  propulsion 
seems  to  flag  and  there  is  a  fresh  period  of  rest.  The 
distention  of  the  tube  by  the  food  and  the  degree  of  tone 
prevailing  in  its  wall  at  the  moment  determine  whether 
or  not  a  peristalsis  shall  be  developed. 

Rhythmic  Segmentation. — This  name  is  given  to  a  type 
of  movement  often  seen  in  the  small  intestine  which 
does  not  definitely  urge  on  the  contents.  A  series  of 
tight  contractions  will  appear  upon  a  loop  with  small, 
slack  pouches  between.  After  a  moment  there  will  be 
relaxation  where  at  first  there  was  contraction  and  con- 
traction in  the  intermediate  zones  which  were  previously 
flaccid.  The  alternation  continues  for  some  time  and 
is  a  relatively  brisk  action  for  smooth  muscle.  Its  effect 
is  to  slip  small  quantities  of  the  food  back  and  forth 
within  a  short  space,  mixing  them  with  the  juices  and 
shifting  their  contact  with  the  absorbing  cells.  The 
massaging  effect  upon  the  blood-vessels  and  lymph  spaces 
of  the  intestinal  wall  may  be  an  important  feature  of 
this  reaction. 

The  Fluid  Exchanges  of  the  Intestine. — During  diges- 
tion the  secretions  enter  the  canal  and  the  water  of  which 


204  HUMAN    PHYSIOLOGY 

they  are  mainly  composed  is  absorbed  again  in  nearly 
equal  volume.  We  probably  have  little  idea  of  the 
amount  of  fluid  thus  passing  in  and  out.  If  all  the  secre- 
tions could  be  drained  away,  and  not  held  for  the  re- 
covery of  the  water,  the  daily  quantity  would  doubtless 
be  more  than  a  gallon — perhaps  very  much  more.  So 
long  as  absorption  fairly  balances  secretion  there  is  no 
great  draft  upon  the  blood  and  tissues.  Such  a  with- 
drawal does  occur  in  diseases  like  cholera  and  in  any 
severe  catharsis. 

If,  on  the  contrary,  absorption  were  to  exceed  secre- 
tion, the  result  would  be.  the  concentration  of  the  in- 
testinal contents  until  the  residue  might  be  reduced  to  a 
crust  upon  the  walls.  This  does  not  occur,  at  least 
in  any  such  extreme  degree.  The  balance  between 
secretion  and  absorption  seems  most  of  the  time  to  be 
nicely  struck. 

The  Pancreatic  Juice. — This  valuable  reagent  makes 
its  appearance  in  the  duodenum  about  the  time  that  the 
first  portions  of  the  gastric  contents  come  through  the 
pylorus.  The  pancreas,  like  the  glands  of  the  stomach, 
is  subject  both  to  nervous  and  to  chemical  influences. 
But,  according  to  the  opinion  generally  held,  the  relative 
importance  of  the  two  is  contrasted  in  the  two  cases: 
nervous  factors  are  dominant  in  the  stomach  while  the 
regulation  of  the  pancreas  is  more  largely  chemical. 
This  is  rather  to  be  expected,  for  the  activity  of  the 
pancreas  is  not  coincident  with  the  pleasure  and  interest 
of  meals  as  is  the  opening  work  of  the  gastric  glands. 

It  is  reported  that  the  introduction  of  acid  into  the 
duodenum  is  followed  within  a  short  time  by  the  flow  of 
pancreatic  juice.  The  injection  of  acid  into  the  blood 
does  not  have  this  effect.  Hence  it  has  been  inferred 
that  the  acid,  striking  into  the  mucous  membrane  of 
the  duodenum,  produces  some  new  agent  which  passes 
through  the  circulation  to  the  pancreas  and  acts  upon  its 
cells  after  the  fashion  of  a  drug.  The  agent  assumed  is 
called  secretin.  We  know  that  the  results  are  not  due  to 


INTESTINAL    DIGESTION  205 

mere  nervous  irritation  by  the  acid  because  it  is  possible 
to  make  an  extract  from  the  duodenal  lining  of  one 
animal  (treating  it  with  acid)  and  to  inject  this  extract 
into  the  blood  of  a  second  animal  which,  thereupon,  se- 
cretes pancreatic  juice.  Secretin  is  an  example  of 
what  we  call  a  hormone,  a  product  added  to  the  blood 
in  one  place  but  having  its  important  effects  elsewhere. 

The  juice  which  flows  from  the  duct  of  the  pancreas 
plays  a  large  part  in  digestion.  It  is  customary  to  say 
that  it  contains  three  enzymes.  One  of  these  is  a 
diastase  or  amylase  .closely  resembling  the  ptyalin  of 
the  saliva.  Under  its  influence  starch  may  be  digested 
from  its  original  condition  or  the  work  may  be  taken 
up  in  the  dextrin  stages  and  carried  forward.  The 
consequences  are  practically  the  same  as  though  the 
saliva  had  resumed  its  action  after  the  interruption 
suffered  in  the  stomach. 

The  pancreatic  juice  contains  also  a  lipase,  an  enzyme 
adapted  to  act  on  fat.  Under  its  influence  fats  undergo 
cleavage  with  the  formation  of  fatty  acids  and  glycerin. 
At  an  early  stage  in  the  intestinal  digestion  of  fats 
emulsification,  that  is,  fine  subdivision,  takes  place. 
This  is  favorable  to  digestion,  as  we  have  already  pointed 
out,  but  is  not  to  be  confused  with  digestion  itself. 
There  is  the  possibility  in  the  intestine,  not  existing  in 
the  stomach,  that  the  fatty  acids  formed  may  combine 
with  alkali.  This  is  the  reaction  known  as  saponifica- 
tion  or  soap  formation.  It  has  proved  very  difficult 
to  say  how  far  it  usually  goes  on. 

Tryptic  Digestion. — Pancreatic  juice,  as  it  comes  from 
the  gland,  may  or  may  not  have  the  power  to  continue 
the  digestion  of  proteins.  If  it  does  not  have  this  power 
at  the  outset  it  is  destined  to  acquire  it  after  mixing  with 
the  bile  and  the  intestinal  juice.  These  secretions  are 
said  to  be  capable  of  activating  the  pancreatic  juice. 
The  protein-splitting  enzyme  of  active  pancreatic  juice 
is  known  as  trypsin.  It  carries  along  the  transformation 
begun  by  pepsin. 


206  HUMAN    PHYSIOLOGY 

Trypsin  differs  from  pepsin  in  two  obvious  ways: 
First,  it  does  not  require  the  cooperation  of  an  acid. 
In  fact,  acid  of  such  a  strength -as  that  in  the  stomach 
would  destroy  the  pancreatic  enzymes.  Second,  it  is 
able  to  advance  the  cleavage  rapidly  beyond  the  stage 
described  as  that  of  the  peptones.  Thus  the  usual 
products  of  peptic  digestion  are  subject  to  further  de- 
composition in  the  intestine  before  they  are  absorbed. 
The  simple  compounds  into  which  the  peptones  are  at 
length  resolved  are  best  called  ammo-acids.  They  have 
been  well  spoken  of  as  "  building  stones,"  for  they  must 
evidently  be  used  in  new  combinations  for  growth  and 
repair. 

The  Intestinal  Juice. — There  is  much  disagreement  as 
to  the  properties  and  importance  of  this  secretion.  It 
is  certainly  abundant  and  the  general  tendency  is  to 
attribute  to  it  a  larger  share  in  the  work  of  digestion  than 
was  formerly  granted.  It  is  supposed  to  change  the 
three  common  sugars  which  are  not  fit  for  direct  intro- 
duction into  the  blood  (cane  sugar,  malt  sugar,  and 
milk  sugar),  into  sugars  of  the  simpler  order  which  are 
acceptable  to  the  body  cells.  It  probably  contains  the 
enzyme  erepsin  which  cooperates  with  trypsin  in  the  later 
stages  of  protein  digestion. 

The  Bile. — The  liver  is  always  secreting  bile.  It  does 
not  have  periods  of  repose  like  those  which  seem  to  be 
enjoyed  by  the  stomach  and  the  pancreas.  But  the 
bile  is  not  always  entering  the  intestine,  for  there  is 
provision  for  its  temporary  storage  in  the  gall-bladder. 
This  is  a  contractile  sac  lodged  in  a  hollow  of  the  under- 
surface  of  the  liver.  It  is  so  connected  with  the  system 
of  ducts  that  the  bile  flowing  from  the  liver  may  either 
pass  to  the  intestine  or  turn  aside  to  gather  in  the  gall- 
bladder. At  another  time  a  large  quantity  of  bile  may 
be  expelled  from  the  bladder  and  introduced  into  the 
alimentary  canal. 

Although  the  secretion  of  the  bile  is  always  in  progress 
it  is  accelerated  after  meals  and  the  hormone,  secretin, 


INTESTINAL    DIGESTION  207 

is  believed  to  work  upon  the  liver  as  well  as  the  pancreas. 
Bile  scarcely  deserves  to  rank  as  a  digestive  juice.  It 
has  little  virtue  by  itself.  But  it  is  certain  that  it 
creates  a  favorable  environment  for  the  action  of  the 
pancreatic  enzymes.  When  it  is  prevented  from  enter- 
ing the  intestine,  as  in  ordinary  jaundice,  the  capacity 
to  digest  and  particularly  to  absorb  food  is  reduced. 
This  is  especially  true  of  fats. 

While  the  presence  of  bile  favors  digestion,  the  secre- 
tion contains  some  constituents  which  are  regarded  as 
wholly  useless  wastes.  This  is  the  character  of  the 
pigments  which  give  bile  its  pronounced  color.  They  are 
chemically  related  to  the  red  coloring  matter  of  the 
blood  and  are  undoubtedly  derived  from  it.  Two  chief 
pigments  are  recognized,  one  which  is  red  and  one  which 
is  green.  The  green  pigment  is  predominant  in  the 
bile  of  most  animals;  human  bile  varies  from  green  to 
orange.  On  the  whole  it  may  be  said  that  bile  occupies 
a  place  intermediate  between  that  of  a  true  digestive 
juice  like  the  gastric,  which  is  secreted  on  occasion,  and 
that  of  the  urine  which  is  a  vehicle  for  waste  and  formed 
continuously. 

The  Colon. — The  value  to  man  of  this  part  of  the 
tract  is  dubious.  •  We  know  that  the  average  rate  of 
progress  for  its  contents  is  slower  than  that  in  the  small 
intestine.  During  a  period  which  is  often  more  than 
twelve  hours  matter  received  from  above  is  retained 
in  this  region.  No  fresh  digestive  juice  of  any  conse- 
quence is  added  to  it  and  the  changes  taking  place  are 
mainly  induced  by  bacteria.  Some  of  the  products  may 
be  available  for  nutrition  if  absorbed  but  others  are 
certainly  injurious.  A  limit  is  set  to  the  bacterial  proc- 
esses by  the  drying  of  the  contents  which  is  a  marked 
feature  of  the  interval  passed  in  the  large  intestine. 
Whatever  questions  may  be  raised  with  reference  to 
its  other  uses,  it  serves  as  a  retriever  of  water. 

In  animals  which  eat  coarse,  woody  food  the  large 
intestine  is  capacious  and  probably  plays  a  considerable 


208 


HUMAN    PHYSIOLOGY 


1P.M. 


Fro.  48, 


20PM. 


INTESTINAL    DIGESTION 


209 


7AM 


8  AM 


FIG.  43. — These  figures  are  intended  to  show  the  probable  advance 
of  the  food  during  the  next  few  hours  after  dinner.  For  the  sake  of 
simplicity  the  tract  is  represented  as  free  from  food  taken  pre- 
viously and  no  supper  is  eaten.  The  course  of  the  small  intestine  is 
diagrammatic. 

At  1  p.m.  the  stomach  is  full  and  active. 

At  3  the  stomach  is  smaller,  having  forwarded  part  of  its  contents  to 
the  small  intestine. 

At  5  the  stomach  is  about  empty  and  digestion  is  in  progress  at  in- 
tervals all  along  the  small  intestine. 

At  10  the  small  intestine  is  clear.  There  is  antiperistalsis  in  the 
ascending  colon.  The  foremost  portion  of  the  material  is  near  the 
spleen.  The  mass  now  consists  less  of  food  than  of  associated 
secretions. 

At  7  a.m.  the  chief  accumulation  is  in  the  sigmoid  flexure.  The  lag- 
ging part  is  near  the  spleen. 

Breakfast  is  eaten,  the  lower  part  of  the  tract  wakes  to  activity 
at  the  same  time  as  the  stomach,  and  at  8  the  sigmoid  has  thrust  its 
contents  to  the  rectum.  This  is  the  occasion  for  defecation. 


part  in  digestion.  In  the  rabbit,  for  example,  it  begins 
in  a  large  pouch  (cecum),  usually  distended  with  soft 
contents  undergoing  a  decomposition  which  is  bacterial 
in  its  nature  but  presumably  productive  of  some  com- 
pounds profitable  to  the  organism.  Such  an  animal 
might  not  absorb  nearly  so  large  a  percentage  of  its  diet 
if  deprived  of  the  colon.  A  dog  or  a  cat  does  very  well 

14 


210  HUMAN    PHYSIOLOGY 

without  this  segment  of  the  canal  and  so  does  a  man- 
Enthusiasts  have  advocated  its  general  abolition  but 
the  surgical  ordeal  is  too  severe  to  be  courted.  Persons 
who  have  lost  the  colon  lose  a  good  deal  of  water  but  no 
substantial  nutriment  in  the  discharges. 

In  the  lower  animals  at  least,  the  ascending  colon  is 
most  of  the  time  swept  by  waves  which  run  counter  to 
what  we  are  inclined  to  call  the  normal  direction.  They 
are  said  to  be  antiperistaltic.  The  backward  thrust 
tends  to  keep  the  food  (now  called  so  only  by  courtesy) 
packed  into  the  first  part  of  the  colon.  Its  return  to  the 
small  intestine  is  hindered  by  a  combination  of  sphincter 
and  mechanical  valve  at  the  junction  of  the  two  divi- 
sions. At  long  intervals  a  strong  contraction  of  the 
cecum  and  ascending  colon  drives  a  mass  of  the  contents 
onward  as  far  as  the  left-hand  half  of  the  transverse 
colon.  Here,  near  the  spleen,  there  is  often  a  period 
of  retention. 

When  this  section  of  the  tube  in  its  turn  is  aroused 
to  react,  a  vigorous  peristalsis  forwards  the  contents  to 
another  place  of  quiescence,  the  sigmoid  flexure.  This 
portion  of  the  colon  is  about  horizontal  when  the  human 
body  is  upright.  There  is  nothing  to  correspond  with 
it  in  the  quadrupeds  and  it  is  looked  upon  as  an  adapta- 
tion to  the  erect  position.  If  the  human  colon  were 
curved  like  that  of  a  cat  any  material  that  passed  the 
spleen  might  be  jarred  down  into  the  rectum  and  dis- 
tend it  disagreeably.  The  sigmoid  receives  and  bears 
up  the  load. 

The  sigmoid  in  most  subjects  thrusts  its  fecal  contents 
into  the  rectum  about  once  in  twenty-four  hours;  in 
many  cases  this  happens  with  clock-like  regularity. 
The  distended  rectum  at  once  begins  to  develop  peris- 
taltic waves  which  the  sphincters  of  the  anus  at  first 
resist.  This  strife  is  attended  with  discomfort  and 
furnishes  the  call  to  defecate.  If  there  is  a  postpone- 
ment the  rectum  may  relax  and  give  no  sense  of  fulness. 
If,  however,  the  time  is  favorable  the  sphincters  may 


INTESTINAL    DIGESTION  211 

be  relaxed  and  the  rectum  emptied  by  its  own  contrac- 
tions reinforced  by  the  voluntary  application  of  pressure 
to  the  abdomen.  When  this  is  in  progress  there  may 
be  a  transference  of  more  fecal  matter  from  the  region 
of  the  spleen  through  the  sigmoid  to  the  rectum  and  this 
second  portion  may  be  evacuated  after  the  original  mass. 

The  Feces. — The  waste  in  the  lower  bowel  is  often 
thought  of  as  a  residue  from  the  diet.  It  may  be  so  in 
part  but  when  the  health  is  good  and  the  food  digestible 
it  consists  more  largely  of  secretions.  The  absorption 
of  food  is  usually  very  nearly  as  good  as  it  can  possibly 
be.  Not  more  than  10  per  cent. — often  not  more  than 
5  per  cent. — of  the  ration,  so  far  as  it  has  theoretic  value, 
is  allowed  to  escape.  It  is  otherwise,  of  course,  when 
there  is  diarrhea. 

Anything  which  is  entirely  indigestible  must  naturally 
be  included  in  the  feces.  The  most  important  substance 
which  has  this  character  is  cellulose,  the  material  which 
is  found  in  the  walls  of  plant  cells.  Fruits  and  coarse 
vegetables  furnish  it  in  large  amounts.  The  digestive 
juices  do  not  appear  to  attack  it;  the  bacteria  of  the 
intestine  may  decompose  it  to  some  extent.  Its  possible 
solution  is  probably  of  no  moment  to  us  so  far  as  the 
cellulose  itself  is  concerned,  but  it  may  be  indirectly 
important  since  the  removal  of  the  envelopes  from  plant 
cells  may  expose  the  proteins  and  starch  to  the  action 
of  the  juices. 

Cellulose  is  a  type  of  what  some  writers  have  called 
" roughage."  It  is  taught  by  most  authorities  that  a 
moderate  amount  of  indigestible  matter  in  the  diet  is 
wholesome.  It  is  supposed  to  stimulate  the  lining  of 
the  canal  by  its  contact  and  to  promote  peristalsis. 
It  may  be  considered  to  act  like  the  sawdust  which  the 
janitor  throws  upon  the  floor  before  sweeping.  As  the 
roughage  is  pushed  along  the  tube  it  gathers  up  and 
takes  with  it  the  less  bulky  but  more  deleterious  wastes. 
The  action  might  be  described  as  a  scouring. 

The    feces    contain    enormous    numbers    of   bacteria, 


212  HUMAN    PHYSIOLOGY 

living  and  dead.  They  also  contain  mucus  and  cast- 
off  cells  from  the  epithelium  of  the  tract.  The  usual 
coloring  matter  is  from  the  bile  but  modified  by  de- 
composition from  its  original  form.  The  gases  of  the 
colon  are  mainly  produced  by  fermentation  processes. 
The  most  offensive  and  doubtless  the  most  poisonous 
compounds  originate  from  proteins,  a  fact  which  sug- 
gests one  reason  for  temperance  in  the  consumption  of 
nitrogenous  foods. 

Absorption. — It  may  have  been  gathered  from  what 
has  been  said  that  the  valuable  part  of  the  food  is  re- 
moved to  the  circulation  before  the  colon  is  reached. 
This  is  normally  true  though  the  colon  has  probably  some 
reserve  power  to  absorb  nutriment.  The  small  in- 
testine occupies  the  central  position  in  the  process; 
absorption  such  as  takes  place  in  the  stomach  is  pre- 
liminary and  that  from  the  colon  supplementary.  It 
used  to  be  customary  to  say  that  no  important  work 
along  this  line  was  done  by  the  stomach  but  that  is  too 
radical  a  statement. 

Some  absorption  of  sugar  and  peptones  may  occur  in 
the  stomach.  The  organ  is,  singularly,  unable  to  take  up 
much  water.  Water  taken  on  an  empty  stomach  seems 
to  pass  the  pylorus  freely  and  is  soon  distributed  along 
the  small  intestine.  When  water  is  taken  with  a  meal 
it  is  said  to  slip  along  the  lesser  curvature  and  to  take  a 
position  in  the  antrum  in  advance  of  the  more  solid 
contents.  Alcohol  is  absorbed  from  the  stomach  with 
great  speed;  it  is  not  necessary  to  wait  for  a  transfer  of 
the  beverage  to  the  intestine  to  obtain  the  cerebral 
reaction. 

The  small  intestine  is  specialized  for  the  duty  of 
absorption.  This  is  apparent  when  we  notice  the  ex- 
tension of  its  surface.  Examination  with  the  naked  eye 
shows  that  this  is  increased  by  the  presence  of  many 
cross-folds.  Under  the  microscope  further  evidence  of 
such  an  extension  is  gained.  The  lining  is  discovered  to 
be  thickly  studded  with  eminences,  the  villi.  These  are 


INTESTINAL    DIGESTION  213 

finger-shaped  processes,  rising  above  the  general  level 
in  contrast  to  the  glands  which  sink  below  it.  The 
arrangement  of  the  villi  suggests  the  bristles  in  a  flat 
brush,  but  their  scale  is  more  like  that  of  the  nap  on 
velvet.  The  result  of  their  existence  is  that  the  number 
of  epithelial  cells  in  contact  with  the  food  is  vastly 
augmented. 

The  details  of  the  act  of  absorption  are  full  of  dif- 
ficulty.    We  cannot  deal  with  them  beyond  emphasizing 


FIG.  44. — Use  is  made  here  of  a  perspective  artifice  as  in  Fig.  8. 
A  bit  of  the  lining  of  the  small  intestine  is  shown  cut  through  and  ex- 
tending away  from  the  observer.  The  eminences  are  villi  with  capil- 
lary nets  inside;  the  slender  pits  are  glands. 

a  few  points.  We  are  to  think  of  the  fluid  contents  of 
the  intestine  on  one  side  of  a  membrane  composed  of 
living  cells.  On  the  other  side  of  the  membrane  is  lymph 
in  the  spaces  of  a  loosely  knit  tissue.  Blood  is  moving 
steadily  through  the  vicinity  in  capillary  vessels  the  walls 
of  which  permit  free  exchanges.  One  might  expect  a 
certain  movement  of  water  and  dissolved  substances  be- 
tween the  blood  and  the  intestine — what  is  colloquially 
called  a  "  soaking  through."  But  this  simple  conception 
does  not  carry  us  far. 


214  HUMAN    PHYSIOLOGY 

We  find  that  the  intestinal  lining  behaves  quite 
differently  from  any  simple  membrane  with  which  it 
might  be  natural  to  compare  it.  The  largest  allowance 
must  be  made  for  its  living  state.  Because  each  of 
its  cells  is  endowed  with  some  of  the  powers  of  an 
organism  we  can  say  that  the  epithelium  has  selective 
capacity.  Energy  is  probably  applied  to  determine  the 
movement  of  the  various  compounds  through  the  cells. 
Certain  bodies  which  are  rated  as  highly  diffusible  by 
ordinary  standards  are  not  received  through  the  mucous 
membrane. 

A  comparison  between  grape  sugar  and  magnesium 
sulphate  (Epsom  salts)  will  make  clear  what  is  meant 
by  selective  absorption.  When  a  test  is  made  with 
parchment  or  any  inert  membrane  the  sugar  is  found 
to  diffuse  less  readily  than  the  magnesium  sulphate. 
The  facts  are  reversed  in  the  intestine;  the  sugar  is 
absorbed  with  ease,  while  the  salt  is  excluded  almost 
entirely  from  the  cells  of  the  epithelial  barrier.  The 
purgative  action  of  the  salt  is  due  to  its  failure  to  be 
absorbed  and  to  the  fact  that  it  keeps  back  from  ab- 
sorption a  large  volume  of  water  required  to  hold  it  in 
solution. 

The  compounds  ordinarily  absorbed  are  water,  certain 
salts  (especially  chlorids),  simple  sugars,  glycerin,  fatty 
acids  or  soaps,  and  amino-acids.  This  statement  means 
that  these  compounds  disappear  from  the  intestine, 
but  it  does  not  strictly  follow  that  they  arrive  in  the 
blood.  There  is  the  possibility  of  changes  affecting 
them  in  their  passage  through  the  cells.  A  striking 
comparison  has  been  made  between  secretion  and  ab- 
sorption. Gland  cells  take  various  materials  from  the 
blood  and  often  manufacture  entirely  new  compounds 
which  they  put  out  from  their  free  border.  Absorbing 
cells  receive  sundry  substances  from  the  cavity  of  the 
intestine  and  work  them  over  before  transferring  the 
resulting  bodies  to  the  interior  of  the  villi.  Such  cells 
may  be  said  to  secrete  inward  instead  of  outward. 


INTESTINAL   DIGESTION  215 

The  best  known  case  of  a  modifying  influence  exerted 
by  the  lining  cells  of  the  intestine  is  their  action  upon 
the  products  of  fat  digestion.  Glycerin  and  fatty 
acids  or  soaps  disappear  from  the  canal  but  the  cells 
work  a  change  which  is  the  reverse  of  digestion  and  that 
which  enters  the  lymph,  and  later  the  blood,  is  neutral 
fat  in  place  of  the  compounds  formed  by  its  cleavage. 

The  history  of  the  foods  after  absorption  can  best  be 
followed  when  we  shall  have  given  some  attention  to  the 
blood  in  which  they  are  represented  and  the  main  facts 
of  the  circulation.  It  may  be  well  to  anticipate  one 
point.  Material  arriving  within  the  villi  must  at  first 
be  in  the  indefinite  lymph  spaces  of  the  tissue.  From 
these  crevices  it  may  enter  the  capillaries — as  most  of  it 
does — or  it  may  leave  the  vicinity  by  pursuing  channels 
of  another  order,  the  lymphatics.  This  latter  course 
is  taken  by  most  of  the  fat. 


CHAPTER  XVI 
THE  BLOOD 

The  blood  has  several  well-defined  services.  It  is 
a  carrier  of  food  and  of  waste.  It  receives  the  food  from 
the  alimentary  canal  and  bears  it  away  to  places  of 
storage  or  to  tissues  where  it  is  to  be  oxidized.  It  re- 
ceives waste  from  the  active  tissues  and  transports  it 
to  the  organs  of  excretion,  especially  the  lungs  and  the 
kidneys,  through  which  it  is  eliminated.  The  lungs 
have  a  double  function  since  the  blood  in  passing  through 
them  not  only  shakes  off  the  chief  oxidized  waste-product 
of  the  body,  carbon  dioxid,  but  gains  oxygen  in  its  place. 

The  blood  is  also  a  carrier  of  compounds  which  can 
hardly  be  classified  as  foods  or  as  wastes,  the  hormones. 
The  conception  of  a  hormone  has  been  suggested  in  con- 
nection with  the  secretin  formed  in  the  epithelial  cells 
of  the  duodenum.  A  hormone  passes  from  the  place 
of  its  origin  to  another  locality  and  modifies  the  be- 
havior of  some  organ  or  tissue.  The  importance  of  such 
agents  is  more  and  more  widely  recognized.  Where 
we  formerly  believed  that  the  nervous  system  furnished 
the  principal  bonds  between  different  parts  of  the  body 
we  now  attribute  to  hormones  many  of  the  influences 
which  clearly  proceed  from  certain  organs  to  others. 

One  service  of  the  circulating  blood  which  is  often 
ignored  is  the  distribution  of  heat.  The  active  tissues 
— those  in  which  oxidation  is  going  on  rapidly — must 
warm  the  blood  which  is  passing  through  them.  This 
transmission  of  heat  to  the  blood  limits  the  rise  of  tem- 
perature in  such  tissues  and  the  blood  imparts  some  of 
the  heat  to  less  active  regions  of  the  body.  It  also" 
carries  heat  to  the  surfaces  of  the  skin  and  the  respiratory 
tract  through  which  it  can  be  dissipated. 

216 


THE    BLOOD  217 

The  Quantity  of  the  Blood. — This  cannot  be  estimated 
by  measuring  the  blood  escaping  from  the  opened  vessels 
of  an  animal,  for  much  will  fail  to  come  out.  The  residual 
blood  may  be  washed  out  and  the  dilution  of  the  mixture 
estimated.  It  will  then  be  possible  to  calculate  the 
original  total.  Earlier  estimates  of  the  blood  in  the 
full-grown  human  body  placed  the  volume  at  5  quarts 
or  liters.  The  recent  tendency  is  to  lower  figures, 
4  liters  or  9  pounds  being  a  reasonable  assumption. 

Plasma  and  Corpuscles. — We  may  call  blood  a  red 
fluid  but  the  color  is  due  to  microscopic  bodies  suspended 
in  a  liquid  which  is,  by  itself,  nearly  colorless.  The 
liquid  basis  is  best  called  the  plasma.  The  suspended 
particles  are  the  " formed  elements"  or  corpuscles.  It 
is  convenient  to  call  them  cells,  though  the  majority  of 
them  do  not  come  up  to  the  standard  in  all  respects. 
If  we  give  them  this  rank  we  can  look  upon  blood  as  a 
tissue,  the  intercellular  substance  being  fluid  instead 
of  solid. 

The  plasma  constitutes  something  more  than  half 
the  volume  of  human  blood.  Considering  that  the 
corpuscles  fill  about  40  per  cent,  of  the  space  in  any  mass 
of  blood  we  may  well  wonder  at  its  free-flowing  properties 
The  solid  bodies  must  be  both  smooth  and  elastic  to 
permit  such  perfect  fluidity.  The  corpuscles  are  defi- 
nitely denser  than  the  'plasma  and  in  some  cases  they  may 
subside  in  a  tall  vessel  until  there  is  a  clear  layer  of 
plasma  at  the  top  from  which  samples  can  be  drawn. 

Blood  plasma  is  a  highly  complex  solution.  The  most 
abundant  of  the  compounds  in  it  are  the  proteins.  The 
plural  is  used  with  sufficient  reason  for  there  is  no  doubt 
that  three  kinds  of  protein  are  present  if  not  a  larger 
number.  The  significance  of  these  proteins  is  obscure. 
Not  long  ago  they  were  thought  to  be  formed  continually 
in  the  cells  lining  the  intestine  and  consumed  as  food  by 
the  tissues  in  general.  But  the  view  is  gaining  acceptance 
that  they  are  a  rather  stable  and  permanent  mass,  little 
subject  to  depletion  and  so  requiring  but  little  renewal. 


218  HUMAN    PHYSIOLOGY 

Non-protein  foods  are  represented  in  the  plasma  but 
more  scantily  than  would  be  expected.  The  sugar  is 
usually  not  much  more  than  one  part  in  a  thousand,  while 
the  fat  content  is  higher  but  still  a  mere  fraction  of  1 
per  cent.  The  salts  of  the  plasma  are  kept  nearly  con- 
stant by  the  reciprocal  adjustment  of  excretion  to  ab- 
sorption. The  conspicuous  one  is  sodium  chlorid  or 
"  common  salt/'  the  only  one  which  we  are  at  pains  to 
add  to  the  diet.  Salts  of  calcium  and  potassium  are 
present  in  very  small  quantities.  It  might  be  thought 
that  their  being  in  the  blood  was  purely  accidental  and 
of  no  moment.  This  is  far  from  being  the  case  for  the 
removal  of  either  calcium  or  potassium  from  the  plasma 
is  most  disturbing  to  many  of  the  activities  of  the  tissues. 

An  interesting  suggestion  has  been  made  that  the 
salts  of  the  plasma  are  the  same  in  variety  and  pro- 
portion as  those  that  existed  in  the  prehistoric  sea. 
The  simpler  marine  animals  might  be  expected  to  have 
their  body  fluids  based  on  sea  water.  Their  descendants 
would  inherit  this  standard  of  composition.  Now  the 
sea  water  of  our  age  is  perhaps  three  times  as  concen- 
trated as  the  blood  of  vertebrate  animals,  but  geology 
teaches  that  it  was  formerly  dilute  and  must  always  be 
gaining  in  salts  as  they  are  washed  from  the  rocks  and 
soil.  It  is  a  fascinating  thought  that  races  of  animals 
may  have  more  power  to  maintain  fixity  in  their  makeup 
than  the  inorganic  world  around  them. 

We  should  naturally  examine  the  plasma  for  the 
presence  of  compounds  clearly  recognizable  as  waste- 
products.  These  are  in  fact  to  be  found  but  in  singularly 
small  amounts.  This  condition  points  to  an  extra- 
ordinary efficiency  on  the  part  of  the  excretory  organs, 
especially  the  kidneys.  One  waste-product  is  indeed 
found  in  relative  abundance;  this  is  carbon  dioxid 
and  it  is  carried  both  in  the  plasma  and  in  the  corpuscles. 
We  infer  the  existence  in  the  plasma  of  many  bodies, 
such  as  hormones,  not  because  we  can  detect  them  by 
chemical  tests,  but  because  the  blood  has  certain  actions 


THE    BLOOD  219 

for  which  it  is  reasonable  to  assume  agents.  It  is  the 
same  with  the  substances  supposed  to  confer  immunity 
against  this  and  that  disease;  we  do  not  recognize  them 
directly,  but  feel  that  they  must  be  there  to  account  for 
observed  facts. 

Red  Corpuscles. — The  great  majority  of  the  formed 
elements  in  the  blood  are  red  corpuscles.  Sometimes 
they  are  called  erythrocytes.  The  deep  red  color  and  the 
opacity  of  blood  depend  on  their  presence,  but  these  are 
not  evident  in  the  single  corpuscle; 
they  result  from  the  superposition 
of  many  layers.  These  corpuscles 
are  individually  minute  discs  with 
slightly  hollowed  surfaces.  When 
they  are  driven  on  by  currents,  as  FIG.  45.— Red  blood- 
in  the  blood-vessels,  they  are  apt  corpuscles.  Several  are 

to  be  cup-shaped,  the  bulging  of  the  *£»  '^S^ 
centers  showing  very  clearly  the  elas-  ters  are  evident.  In  one 

tic  nature  of  the  body.  case  two  corpuscles  are 

mi          •  £  Ji  i  i  overlapped  to  snow  that 

The  size  of  the  red  corpuscles  in  they  are  transparent. 
any  given  animal  species  is  remark-  They  tend  to  run  into 
ably  constant.  Most  cells  vary  p.nf  •  asA  shown  **  th^ 

.  ,        ,  ,       ,  .        right.     A  saucer-shaped 

considerably  but  these  are  as  uni-     form  is  represented, 
form  as  coins  stamped  by  the  same 

die.  The  red  corpuscle  of  man  measures  Jli200  incn 
across  its  face.  Its  thickness  is  about  one-fifth  as 
great.  When  blood  is  viewed  under  the  microscope 
the  enormous  number  of  the  corpuscles  is  at  once  sug- 
gested. It  can  be  determined  quite  accurately  by 
careful  dilution  and  counting  the  elements  in  a  small, 
measured  volume.  The  conclusion  reached  is  that  in  the 
original  blood  there  are  about  5,000,000  corpuscles  in  a 
cubic  millimeter  (the  space  occupied  by  a  coarse  grain  of 
sugar).  The  whole  number  in  the  body  must  be  some- 
thing like  4,000,000  times  this  large  number  (20  trillion). 
When  we  compare  the  microscopic  appearance  of  blood 
from  different  animals  we  find  that  the  size  of  the  cor- 
puscles does  not  correspond  at  all  with  the  size  of  the 


220  HUMAN    PHYSIOLOGY 

organism.  The  largest  are  found  in  small  reptilian 
or  amphibian  forms,  the  smallest  in  mammals  of  good 
size.  In  general,  the  corpuscles  of  the  warm-blooded  are 
smaller  than  those  of  the  cold-blooded  types.  This 
appears  meaningless  until  the  function  of  the  corpuscles 
is  stated.  Their  particular  service  is  to  absorb,  carry, 
and  deliver  oxygen.  For  such  a  purpose  small  cor- 
puscles in  great  numbers  must  be  superior  to  large  cor- 
puscles in  smaller  numbers.  The  characteristic  of 
the  warm-blooded  animal  is  the  high  oxygen  requirement. 
Moreover,  if  the  corpuscles  are  small,  the  network  of 
vessels  through  which  they  are  sent  can  be  finely  divided 
and  bring  them  close  to  all  the  cells  to  which  they  are  to 
minister. 

It  is  the  total  surface  of  the  red  corpuscles  which  counts 
in  the  execution  of  their  specific  function.  As  a  result  of 
their  vast  number  and  their  shape  the  aggregate  surface 
is  well  nigh  incredible.  Calculations  have  placed  it  as 
high  as  %  acre  for  one  human  being.  An  able 
writer  has  made  this  area  easy  to  visualize  by  saying 
that  it  equals  four  baseball  diamonds.  This  does  not 
mean  that  four  diamonds  could  be  covered  by  the  cor- 
puscles from  the  blood  of  one  man  for  we  are  reckoning 
both  sides  and  allowing  for  the  edges.  Still,  it  is  prob- 
able that  the  corpuscles  would  make  a  continuous  film 
over  a  plot  of  ground  100  feet  square.  It  would  be 
too  delicate  to  redden  the  surface  or  to  be  apparent  in 
any  way. 

The  chief  solid  in  the  red  corpuscles  is  called  hemo- 
globin. This  is  a  protein  and  of  unusual  complexity 
even  for  a  representative  of  that  class  of  compounds.  In 
addition  to  the  five  elements  we  expect  to  find  in  proteins 
generally  (carbon,  oxygen,  nitrogen,  hydrogen,  and 
sulphur)  the  red  pigment  of  the  blood  is  exceptional  in 
containing  iron.  The  percentage  is  low  but  there  is 
iron  enough  in  the  blood  of  a  man  to  make  a  small  nail. 
The  popular  notion  according  to  which  iron  and  "good 
red  blood"  are  connected  has  some  basis  in  fact. 


THE   BLOOD  221 

Hemoglobin  can  be  dissolved  in  water  or  in  plasma  but 
it  is  normally  retained  in  the  corpuscles  through  the 
agency  of  their  other  components.  The  structure 
involved  is  not  well  understood. 

It  is  hemoglobin  which  confers  on  the  corpuscles 
their  power  to  unite  with  oxygen.  The  union  takes 
place  in  the  lungs  and  a  temporary  compound  is  formed 
which  is  called  oxyhemoglobin.  This  is  bright  red.  As 
the  blood  flows  through  active  tissues  close  to  cells 
which  are  consuming  oxygen  the  corpuscles  yield  more 
or  less  of  the  oxygen  which  they  have  just  now  attached. 
In  so  far  as  they  do  this  their  oxyhemoglobin  is  changed 
to  what  is  spoken  of  as  reduced  hemoglobin.  This  is 
blue-black  and  the  more  of  it  there  is  present  the  darker 
the  blood.  It  is  not  usual  for  any  portion  of  the  blood 
to  give  up  all  its  oxygen  and  so  contain  nothing  but 
reduced  hemoglobin.  This  may  happen  in  suffocation 
or,  locally,  in  intense  muscular  activity.  The  details 
are  better  taken  up  in  the  discussion  of  respiration. 

The  History  of  the  Red  Corpuscle. — It  has  been  said 
that  red  corpuscles  are  not  cells  in  the  full  sense  of  that 
term.  It  is  a  question  whether  we  ought  to  consider 
them  to  be  alive;  perhaps  we  gain  nothing  by  assuming 
that  they  are  so.  But  each  corpuscle  is  probably  to  be 
regarded  as  a  modified  or  degenerate  cell  and  its  history 
is  fairly  clear.  It  had  its  origin  in  an  unexpected 
locality,  the  red  marrow  of  the  bones.  We  must  make 
a  distinction  between  this  type  of  marrow  and  the  more 
conspicuous  white  or  yellow  marrow  which  is  found  in 
the  hollow  shafts  of  such  bones  as  those  in  the  arms  and 
legs.  White  marrow  is  largely  fat.  The  red  marrow  is 
found  in  minute  spaces  in  the  expanded  ends  of  the  long 
bones,  for  example,  about  the  knees  and  elbows. 

Microscopic  examination  and  chemical  tests  of  the 
red  marrow  show  that  it  is  composed  of  cells  which  are 
rich  in  hemoglobin.  The  blood  flows  among  these  cells 
and  comes  directly  into  contact  with  them,  since  it  is 
not  here  confined  in  definite  vessels.  Those  cells  which 


222  HUMAN    PHYSIOLOGY 

lie  closest  to  the  passing  stream  are  steadily  evolved  into 
red  corpuscles  and  when  the  transformation  is  complete 
they  detach  themselves  and  drift  away  in  the  current. 
The  original  cells  of  the  red  marrow  have  nuclei  but  none 
can  be  discovered  in  the  mature  corpuscles.  The  hollow 
centers  strongly  suggest  the  loss  which  has  been  suffered. 
It  is  a  curious  fact  that  after  severe  hemorrhage,  when 
the  system  is  taxed  to  restore  the  normal  condition, 
corpuscles  with  nuclei  are  often  to  be  found  in  the  blood. 
Apparently,  at  such  a  time,  corpuscles  not  fully  developed 
are  impressed  into  service. 

The  evidence  goes  to  show  that  there  is  a  fairly  active 
formation  of  red  corpuscles  and  we  must  suppose  that 
there  is  a  corresponding  disintegration.  Whether  this 
occurs  here  and  there  all  over  the  body  or  in  particular 
places  has  been  much  discussed.  Long  ago  it  was 
maintained  that  the  spleen  is  concerned  in  the  work  of 
destruction.  The  view  fell  into  disfavor  but  has  lately 
been  revived  in  a  modified  form.  Removal  of  the  spleen 
in  certain  cases  of  anemia  has  proved  beneficial,  and  it 
is  natural  to  explain  such  an  observation  by  concluding 
that  the  spleen  had  been  destroying  the  corpuscles  more 
rapidly  than  the  loss  could  be  made  good. 

It  was  stated  in  the  previous  chapter  that  the  pigments 
of  the  bile  are  derived  from  the  coloring  matter  of  the 
blood.  Wherever  the  dissolution  of  the  corpuscles  takes 
place  we  must  suppose  that  certain  products  are  carried 
in  the  plasma  and  sooner  or  later  worked  over  by  the 
cells  of  the  liver  with  the  result  that  these  waste-sub- 
stances are  separated.  It  is  noteworthy  that  the  pig- 
ments of  the  bile  do  not  contain  iron ;  this  element  seems 
to  be  treated  as  a  precious  material  which  is  not  to  be 
discarded.  It  is  natural  to  assume  that  by  far  the  larger 
part  of  the  dissolved  matter  yielded  by  the  decomposing 
red  corpuscles  finds  its  way  back  to  the  bone  marrow  to 
be  wrought  into  new  elements. 

White  Corpuscles. — When  one  examines  blood  under 
the  microscope  it  is  possible  to  detect  here  and  there 


THE    BLOOD  223 

among  the  host  of  red  corpuscles  bodies  of  a  different 
type.  These  are  the  white  or,  better,  colorless  cor- 
puscles. There  may  be  one  of  these  to  a  thousand  reds. 
Several  kinds  are  recognized  and  the  proportion  existing 
between  them  is  of  interest  to  the  practitioner.  Of  all 
it  may  be  said  that  they  are  free  from  hemoglobin  and 
that  they  are  complete,  nucleated  cells.  We  speak 
confidently  of  them  as  living.  Some  originate  along 
with  the  red  corpuscles  in  the  bones,  others  in  kernels  of 
tissue  known  as  lymph  nodes,  of  which  more  will  be 
said. 

The  majority  of  the  white  cells  are  of  the  ameboid  type 
to  which  reference  was  made  in  Chapter  IV.  There  we 
indicated  the  power  which  these  cells  have  to  make  their 
escape  from  the  capillaries  and  their  capacity  for  devouring 
bacteria.  Other  services  than  this  have  been  conjectured 
but  without  very  tangible  evidence.  Thus  it  has  been 
thought  possible  that  the  white  cells  of  the  blood  have 
to  do  with  the  assimilation  and  working  over  of  the  foods 
to  adapt  them  to  the  requirements  of  the  tissues. 

Blood-plates. — When  blood  has  been  prepared  by 
special  methods  for  microscopic  study  there  may  be 
found  in  it  quite  numerous  bodies  of  a  smaller  size  than 
either  the  red  or  the  white  corpuscles.  These  are  the 
blood-plates.  They  were  formerly  held  to  be  mere 
particles  of  debris  but  the  belief  is  now  general  that  they 
are  perfect  though  unusually  minute  cells.  They  are 
remarkably  perishable  and  stand  in  a  certain  relationship 
to  a  curious  property  of  blood,  its  coagulability.  This 
must  be  briefly  discussed. 

Coagulation. — Blood  in  the  vessels  is  quite  free-flow- 
ing; it  is  probably  less  viscous  than  is  commonly  sup- 
posed, for  we  are  apt  to  see  it  when  it  is  approaching 
coagulation.  The  capacity  which  it  exhibits  to  set  into  a 
jelly  when  shed  has  a  manifest  value.  It  lessens  and 
often  completely  checks  hemorrhage  by  sealing  over  the 
cut  surface.  There  are  individuals  whose  blood  does 
not  coagulate  and  they  are  in  grave  danger  of  bleeding 


224  HUMAN    PHYSIOLOGY 

to  death  from  slight  wounds.  Their  condition  is  known 
as  hemophilia  and  it  is  inherited  in  certain  lines  of 
descent.  The  clotting  of  blood  upon  injured  surfaces  has 
a  secondary  function  since  it  gives  the  basis  of  the  crust 
or  scab  beneath  which  the  healing  processes  may  go  on. 

When  blood  coagulates  in  a  beaker  the  whole  mass 
appears  for  a  time  as  a  solid  which  gradually  becomes 
more  tenacious.  In  the  course  of  some  hours  it  con- 
tracts and  a  clear  or  but  slightly  stained  fluid  exudes. 
The  distribution  of  the  coloring  shows  that  the  cor- 
puscles are  in  the  clot  and  one  might  infer  that  the  liquid 
separating  must  be  plasma.  It  is  better  to  call  it  by  the 
special  name  of  serum.  Most  of  the  materials  which 
were  originally  in  the  plasma  remain  in  the  serum,  but 
there  is  an  important  exception:  something  has  been 
taken  from  the  plasma  to  knit  the  corpuscles  together 
and  form  the  clot. 

The  central  fact  in  coagulation  is  the  generation  of  a 
gummy  compound  which  is  called  fibrin.  The  absolute 
quantity  of  this  new  substance  is  very  small  but  its 
physical  nature  is  such  that  is  capable  of  changing  a 
liquid  to  a  solid  much  as  gelatin  might  do.  In  the  ab- 
sence of  corpuscles  the  clot  would  have  little  firmness; 
they  give  it  body  and  coherence.  We  have  now  to 
inquire  as  to  the  source  of  the  fibrin  and  why  it  is  formed 
at  particular  times  rather  than  at  others. 

Fibrin  is  a  protein.  It  is  derived  from  another  protein, 
fibrinogen,  which  exists  in  normal  plasma.  Fibrinogen 
is  soluble  and  therefore  its  presence  does  not  attract 
attention;  fibrin  is  relatively  insoluble  and  hence  con- 
spicuous in  its  effects.  The  formation  of  the  insoluble 
fibrin  from  the  soluble  fibrinogen  is  an  instance  of  enzyme 
action  and  may  recall  the  curdling  of  milk  by  gastric 
juice.  The  enzyme  which  causes  the  fibrin  to  form  and  so 
brings  about  the  coagulation  of  blood  is  called  thrombin. 
The  enzyme  comes  into  existence,  or  at  least  becomes 
effective,  under  just  such  conditions  as  attend  the 
shedding  of  blood. 


THE   BLOOD  225 

The  circumstances  which  hasten  and  those  which 
retard  the  clotting  of  the  blood  has  been  studied  in  the 
utmost  detail.  It  might  be  inferred  that  exposure  to  air 
would  be  an  influential  one  but  this  can  hardly  be 
claimed.  Much  more  important  is  contact  with  foreign 
surfaces,  and  all  surfaces  other  than  the  lining  of  the 
blood-vessels  must  be  regarded  as  foreign.  When  blood 
•flows  from  a  cut,  it  passes  over  cells  which  it  does  not 
normally  bathe  and  often  over  those  which  have  been 
torn  or  crushed  so  as  to  yield  peculiar  subtances  of  their 
own.  Then  it  runs  upon  the  dead  skin  and  perhaps 
upon  clothing  or  hair.  These  harsh  and  extended  con- 
tacts favor  coagulation.  Blood  will  not  soon  clot  in  a 
beaker  which  has  been  oiled;  this  measure  seems  to 
create  a  surface  more  like  the  natural  lining  of  the 
system. 

How  are  we  to  connect  foreign  contacts  with  the  pro- 
duction of  thrombin  and  so  of  fibrin?  To  make  a  very 
long  story  short  we  may  say  that  the  damaging  effect 
of  the  strange  surfaces  is  first  felt  by  the  blood-plates. 
As  these  disintegrate  their  constituents  must  pass  into 
solution  in  the  plasma.  Agents  from  this  source  help 
to  perfect  or  render  effective  the  enzyme  thrombin. 
The  formation  of  thrombin  does  not  occur  in  the  absence 
of  calcium  (lime)  salts  in  the  solution  and,  accordingly, 
a  simple  way  of  warding  off  coagulation  is  to  remove  the 
lime  salts  from  fresh  blood  by  adding  a  small  amount  of 
potassium  oxalate.  Other  cells  than  the  blood-plates — 
for  example,  those  of  the  lacerated  tissues  outside  the 
vessels — may  contribute  compounds  favorable  to  the 
coagulation  process. 

From  what  has  been  said  it  will  be  anticipated  that 
clotting  may  take  place  in  the  vessels  if  conditions  arise 
there  which  cause  the  destruction  of  blood-plates.  An 
injury  to  an  artery  or  vein,  as  by  tying  or  clamping,  may 
render  the  lining  so  abnormal  that  it  is  equivalent  to  a 
foreign  surface.  Fibrin  formation  will  then  begin  and 
the  clot  will  build  up  upon  the  injured  area  as  a  founda- 

15 


226  HUMAN    PHYSIOLOGY 

tion  until  it  may  block  the  channel.  The  obstruction  of 
a  blood-vessel  may  be  fatal  if  it  is  in  the  brain  or  the 
heart  but  in  many  cases  it  has  no  serious  results.  This  is 
because  most  parts  of  the  body  are  not  dependent  upon 
single  sources  of  blood-supply. 

Sometimes  a  clot  formed  in  one  place  is  later  detached 
and  moves  to  another  locality  where  it  may  do  more 
damage  than  it  did  in  the  first  region.  Thus  during 
convalescence  from  pneumonia  there  is  the  ugly  possi- 
bility that  blood-clots  from  the  temporarily  obstructed 
veins  of  the  lungs  may  work  loose  and  be  carried  away 
in  the  stream.  One  of  these  may  reach  an  important 
artery  of  the  brain  and  by  stopping  it  cause  sudden  death. 
The  course  followed  in  such  instances  will  be  more  clear 
after  the  next  chapter. 


CHAPTER  XVII 
THE  COURSE  AND  PHYSICS  OF  THE  CIRCULATION 

To  serve  its  various  functions  blood  must  be  kept  in 
motion.  The  pump  which  maintains  the  flpw  is  the 
heart  and  the  flow  itself  is  called  the  circulation  because 
the  blood  is  sent  out  only  to  return  again  and  again. 
In  warm-blooded  animals  the  heart  is  divided  into  right 
and  left  halves  and  is  best  thought  of  as  a  pair  of  force 
pumps  making  simultaneous  strokes.  On  either  side 
there  is  a  chamber  above  called  an  auricle  which  receives 
incoming  blood  and  transmits  it  to  a  second  chamber 
below  called  a  ventricle.  The  ventricles  are  the  chief, 
features  of  the  heart  from  the  standpoint  of  energy 
evolved  and  applied  to  driving  the  blood. 

Vessels  which  conduct  blood  toward  the  heart  are 
called  veins.  It  will  be  seen  that  they  lead  more  or  less 
directly  to  the  two  auricles.  Vessels  which  carry  blood 
away  from  the  heart  are  called  arteries.  They  are 
derived  from  the  ventricles,  the  smaller  ones  springing 
from  the  larger.  If  we  trace  the  course  of  the  blood 
along  the  veins  we  find  it  entering  larger  and  larger 
channels  formed  by  the  union  of  those  which  are  more 
slender  and  more  numerous.  If,  in  the  same  way,  we 
follow  the  blood  in  the  arteries  we  find  it  introduced  into 
more  and  more  numerous  but  finer  branches.  Both 
systems  are  tree-like,  but  in  the  veins  the  flow  is  from 
smaller  to  larger  and  in  the  arteries  from  larger  to  smaller 
divisions. 

Blood  which  goes  out  from  the  left  side  of  the  heart 
will  return  next  to  the  right.  The  arteries  which  are 
supplied  from  the  left  ventricle  empty  into  veins  which 
lead  to  the  right  auricle.  The  connection  between  the 

227 


228 


HUMAN    PHYSIOLOGY 


FIG.  46.— In  this 
diagram,  as  is  usual 
in  such  cases,  right 
and  left  are  reversed. 
This  is  as  though  the 
observer  were  look- 
ing at  another  sub- 
ject. The  short  pul- 
monary path  is  to 
be  traced  from  the 
right  ventricle  to  the 
left  auricle  (P.  C.). 
Alternative  routes 
are  suggested  for  the 
passage  of  the  blood 
through  the  greater 
circulation  from  the 
left  ventricle  to  the 
right  auricle.  The 
blood  which  tra- 
verses the  digestive 
tract  (Z>)  passes 
through  a  second  set 
of  capillaries  in  the 
liver  (L)  before  it 
can  return  to  the  heart, 
arterial  supply  of  blood. 


smallest  arteries  and  the  finest  veins 
is  through  the  capillaries,  microscopic 
channels  with  the  most  delicate  enclos- 
ing walls  that  can  be  imagined.  A 
capillary  may  not  be  much  wider  than 
a  single  red  corpuscle.  But  as  these 
vessels  are  the  narrowest  of  all  they 
are  likewise  by  far  the  most  numerous. 

The  left  ventricle  sends  blood  to  all 
parts  of  the  body.  When  it  comes 
back  to  the  right  side  of  the  heart  it 
has  parted  with  some  of  its  oxygen 
in  the  service  of  the  tissues.  The 
right  ventricle  sends  it  to  the  lungs. 
In  these  organs  the  corpuscles  are 
freshly  charged  with  oxygen  and  some 
carbon  dioxid  is  discharged.  The 
blood  is  returned  to  the  left  side  of  the 
heart  and  is  ready  to  go  out  again  to 
sustain  the  activities  of  the  various 
systems.  The  arrangement  is  such 
that  every  corpuscle  which  has  made 
the  journey  from  the  left  side  of  the 
heart  to  the  right  is  compelled  to  pass 
through  the  capillaries  of  the  lung  tis- 
sue before  it  can  go  anywhere  else. 

It  is  somewhat  different  with  cold- 
blooded animals.  These  have  but  one 
ventricle.  Blood  goes  out  from  it  and 
is  sent  in  part  to  the  body  at  large  and 
in  part  to  the  lungs  or  gills.  Only  a 
fraction  of  the  blood  is  fully  oxy- 
genated. But  this  fraction  reenters 
the  heart  and  raises  the  average  com- 
position of  the  mixed  blood.  Thus  a 
standard  is  maintained  which  is  ade- 


Note  that  the  liver  has  in  addition  a  separate 


THE  COURSE  AND  PHYSICS  OF  THE  CIRCULATION      229 

quate  for  the  support  of  life  in  such  animals  but  inferior 
to  that  required  by  the  warm-blooded. 

The  blood  that  is  traversing  the  body  in  general  on  its 
way  from  the  left  side  of  the  heart  to  the  right  is  said  to 
be  in  the  greater  or  systemic  circulation.  That  which  is 
passing  from  the  rijht  sifle  $f  th«  hue^rt  J^ck  t§  th«  kft 
by  way  of  the  lungs  is  said  to  be  in  the  lesser  or  pulmonary 
circulation.  More  than  three-quarters  of  all  the  blood  is 
probably  in  the  systemic  circulation  at  any  given  moment 
But  it  is  also  true  that  the  two  ventricles  pump  out  equal 
quantities  of  blood  in  equal  times.  Students  often  find 
difficulty  in  reconciling  these  two  facts,  but  there  should 
be  none.  It  is  only  necessary  to  reflect  that  no  more 
blood  can  be  pumped  out  from  either  ventricle  than  the 
other  ventricle  supplies  to  it.  The  service  of  the  left 
ventricle  is  much  heavier  than  that  of  the.  right.  It  has 
no  more  blood  to  drive  forth  into  the  arteries  at  each  beat 
but  a  much  greater  mass,  upon  a  longer  journey,  has  to 
be  kept  in  motion. 

There  is  a  somewhat  unfortunate  difference  between  the 
accepted  significance  of  the  nouns  artery  and  vein  and 
that  of  the  adjectives  arterial  and  venous.  Arteries  and 
veins,  as  we  have  indicated,  are  distinguished  by  the 
direction  of  the  current  within  them  with  reference  to 
the  heart.  But  the  adjective  arterial,  applied  to  blood, 
means  " fully  oxygenated"  while  venous  means  " de- 
ficient in  oxygen."  The  systemic  arteries  carry  arterial 
blood  but  the  pulmonary  arteries  contain  blood  which  is 
venous.  The  systemic  veins  convey  venous  blood  while 
that  in  the  pulmonary  veins,  having  just  left  the  lungs, 
is  arterial. 

Arteries  are  elastic  tubes  of  great  strength.  Their 
walls  are  of  considerable  thickness.  An  important 
feature  of  the  smaller  ones  is  a  marked  development  of 
muscle  of  the  smooth  variety.  Veins  are  less  elastic 
than  arteries,  less  muscular,  and  more  capacious.  An 
artery  is  approximately  circular  in  cross-section, 
while  a  vein  is  apt  to  be  elliptic.  It  follows  that  any 


230  HUMAN    PHYSIOLOGY 

unusual  pressure  inside  a  vein  will  round  it  up  and 
greatly  add  to  its  capacity.  It  has  been  said  that  the 
capillaries  are  composed  of  one  layer  of  the  thinnest 
kind  of  epithelium;  this  same  tissue  is  continued  as  a 
lining  through  the  arteries,  the  veins,  and  the  cavities  of 
the  heart.  If  .-£,11  the  connective  and  muscular  tissue 
could  be  removed  from  the  circulatory  system  it  would 
be  quite  completely  mapped  in  epithelium  of  inconceiv- 
able delicacy. 

The  Velocity  of  Blood-flow  in  Vessels  of  Different 
Classes. — Before  we  analyze  the  action  of  the  heart  we 
will  consider  the  main  facts  about  the  progress  of  the 
blood  through  the  vessels.  The  left  ventricle  thrusts  it 
first  into  the  great  main  artery,  the  aorta.  This  rises 
above  the  heart,  arches  over,  giving  off  branches  to  the 
head,  arms,  and  chest,  and  then  sweeps  down  in  front  of 
the  spinal  column.  It  pierces  the  diaphragm  and  supplies 
the  abdominal  organs,  the  trunk  muscles  at  this  level, 
and  finally  the  legs.  The  blood  advances  along  the 
aorta  at  a  high  velocity.  In  one  second  a  corpuscle  may 
move  from  the  root  of  the  vessel  to  a  point  as  much  as  12 
inches  away.  In  all  the  large  arteries  the  blood  moves 
rapidly,  but  nowhere  so  fast  as  at  the  very  beginning. 
The  student  is  apt  to  leap  to  the  conclusion  that  the 
aortic  velocity  is  maximal  because  the  blood  is  there 
under  the  unspent  impetus  of  the  heart's  contraction. 
This  is  a  wrong  notion  as  will  be  shown. 

The  rate  of  flow  in  the  capillaries  can  be  observed 
directly  with  the  microscope  in  various  transparent 
structures,  such  as  the  mesentery.  It  is  very  slow.  A 
corpuscle  is  likely  to  take  fully  a  second  to  traverse  a 
capillary  J^5  inch  in  length.  Such  a  velocity  is  not 
over  ^soo  of  that  in  the  aorta.  In  the  veins  we  find 
the  blood  speeding  up  and  attaining  a  rate  which  is 
not  much  less  than  that  in  the  arteries.  The  highest 
speed  anywhere  in  the  systemic  veins  is  probably  in  the 
two  large  veins  which  lead  nearly  all  the  blood  into  the 
right  auricle.  These  are  the  vence  cavce,  the  superior 


THE  COURSE  AND  PHYSICS  OF  THE  CIRCULATION      231 


draining  the  head,  arms,  and  chest,  and  the  inferior  the 
rest  of  the  body. 

Why  does  the  velocity  fall  from  a  maximum  in  the 
aorta  to  a  minimum  in  the  capillaries  and  then  augment 
again  along  the  course  of  the  veins?  The  underlying 
principle  is  simple  if  the  student  will  not  allow  himself  to 
be  led  away  from  it.  It  is  merely  that 
in  any  stream  the  velocity  is  greatest 
where  the  cross-section  of  the  channel 
is  least  and  lowest  where  it  is  greatest. 
When  the  application  is  made  we  find 
that  we  are  required  to  regard  the 
aorta  as  the  narrowest  and  the  capil- 
laries as  the  widest  part  of  the  sys- 
tem. This  is  not  readily  admitted 
until  the  enormous  number  of  the 
capillaries  is  emphasized.  It  is  not  a 
few  capillaries  but  millions  combined 
which  we  have  to  compare  with  the 
aorta.  It  seems  to  be  a  general  truth 
that  when  a  vessel  forks  the  sum  of 
the  cross-sections  of  the  branches  is 
greater  than  that  of  the  parent  stock. 
So  if  an  artery  divides  the  velocity  will 
be  reduced  while  there  will  be  a  quick- 
ening at  the  point  where  two  veins 
unite  to  make  one.  If  the  velocity  in 
the  veins  never  equals  that  in  the  aorta 
it  is  simply  because  the  two  venae  cavse  have  unitedly 
a  somewhat  larger  cross-section  than  the  great  artery. 

The  Facts  of  Blood-pressure. — A  vein  is  easily 
flattened  under  the  finger;  an  artery  offers  a  strong  re- 
sistance. We  have  here  the  sign  of  a  great  difference 
between  the  pressure  exerted  by  the  blood  in  the  two 
vessels.  The  difference  is  shown  still  more  strikingly 
when  an  artery  and  a  vein  are  cut ;  the  blood  springs  from 
the  artery  in  a  pulsating  jet  while  the  flow  from  the  vein 
is  copious  but  easily  checked.  We  say  that  arterial  pres- 


FIG.  47.— If  a  ves- 
sel (a)  divides  into 
two  branches,  (6)  and 
(c),  these  will  be  in- 
dividually of  less 
cross-section  than  the 
main  trunk  but 
unitedly  they  will  ex- 
ceed it.  Linear  ve- 
locity will  be  lower 
in  the  branches  than 
in  the  parent  stock. 


232  HUMAN    PHYSIOLOGY 

sure  is  high  and  venous  pressure  low.  The  contrast 
became  apparent  to  the  pioneer  investigator  Hales  when 
he  performed  a  certain  memorable  experiment  nearly  two 
centuries  ago. 

Hales  established  a  connection  between  the  femoral 
artery  of  a  prostrate  horse  and  a  vertical  glass  tube  9 
feet  high.  The  blood  mounted  in  the  tube  to  a  height 
of  about  8  feet.  The  column  then  rose  and  fell  by  a 
few  inches  in  the  rhythm  of  the  heart-beat  and  showed 
other  fluctuations  but  in  general  remained  at  this  high 
level.  When  a  vein  was  placed  in  communication  with 
a  similar  vertical  tube  the  blood  rose  only  a  few  inches 
above  the  vessel.  We  have  now  to  explain  the  high  pres- 
sure in  the  artery  and  the  trifling  pressure  in  the  vein. 

The  high  pressure  in  the  aorta  and  its  branches  is  an 
indication  of  the  power  which  the  contracting  ventricles 
has  impressed  upon  the  blood.  If  we  find  a  liquid 
passing  along  a  tube  we  must  conclude  that,  whatever 
the  pressure  it  exerts  at  a  given  point,  a  still  higher 
pressure  prevailed  at  the  source  from  which  it  sprang. 
So  if  we  find  blood  exerting  a  great  pressure  in  an  artery 
we  have  to  assume  that  the  pressure  was  still  greater  in 
the  larger  vessel  of  which  this  is  a  branch  and  highest  of 
all  in  the  left  ventricle  which  gave  the  initial  thrust. 

The  physiologist  of  the  present  day,  if  he  wishes  to 
measure  the  pressure  of  the  blood  in  an  artery,  does  not 
make  use  of  the  tall  and  inconvenient  tube  of  Hales. 
Instead  he  introduces  a  glass  nozzle  into  the  vessel  and 
opens  communication  through  this  with  a  U-shaped  tube 
containing  mercury.  The  pressure  then  forces  the  mer- 
cury down  in  the  arm  next  to  the  artery  and  up  in  the 
distant  one.  A  difference  of  5  or  6  inches  between  the 
levels  of  the  two  ends  will  hold  back  the  blood  in  the 
arteries  of  a  dog  or  cat  and  the  pressure  is  read  in  terms 
of  the  mercury  column,  usually  expressed  in  millimeters. 
A  float  on  the  surface  of  the  mercury  in  the  remote  arm 
of  the  U-tube  is  commonly  arranged  to  write  a  record 
upon  a  travelling  smoked  surface  (Fig.  48). 


THE  COURSE  AND  PHYSICS  OF  THE  CIRCULATION      233 

We  are  in  possession  of  the  following  facts :  pressure  in 
the  arteries  is  high  and  fluctuating,  slightly  higher  in 
the  main  trunks  than  in  their  branches;  pressure  in  the 
veins  is  low  and  relatively  constant.  It  must  be  rather 
higher  in  the  small  veins  than  in  the  large  ones  which 
they  unite  to  form;  the  fact  that  the  blood  flows  from  the 
smaller  to  the  larger  ones  makes  this  certain.  An  im- 


FIG.  48. — The  modern  method  of  measuring  blood-pressure  in  ani- 
mals.' A  glass  nozzle  or  cannula  like  (c)  is  tied  into  an  artery  as  at  (a). 
The  pressure  elevates  the  more  distant  limb  of  a  mercury  column  in  a 
U-tube  and  a  float  on  the  mercury  causes  a  writing-point  to  trace  upon 
a  revolving  drum.  Transmission  from  the  blood  to  the  mercury  is 
by  tubing  filled  with  a  solution  restraining  coagulation. 

portant  inference  from  all  this  is  that  a  great  fall  of 
pressure  occurs  in  the  region  of  the  small  vessels — the 
capillaries  and  the  minute  arteries  and  veins  immediately 
adjoining  them.  This  we  must  presently  account  for. 

We  may  think  of  the  pressure  of  the  blood  as  a  measure 
of  the  energy  applied  to  the  blood  by  the  ventricle.  What 
becomes  of  this  energy?  In  any  such  system,  whether 


234  HUMAN    PHYSIOLOGY 

natural  or  artificial,  it  is  turned  to  heat  by  friction  within 
the  stream  and  along  the  enclosing  walls.  Friction  is 
greater  at  particular  places  and  at  these  same  places  pres- 
sure will  be  cut  down.  The  great  reduction — "loss  of 
head,"  as  the  hydraulic  engineer  would  call  it — occurs  in 
the  blood-vessels  where  the  extensive  subdivision  of  the 
stream  creates  the  greatest  resistance  to  progress.  In  the 
large  tubes,  whether  arteries  or  veins,  there  is  little  re- 
sistance and  only  a  gradual  diminution  of  pressure. 
When  the  returning  blood  nears  the  right  auricle  the 
pressure  is  scarcely  above  that  of  the  atmosphere  and  we 
may  say  that  the  original  impetus  is  spent. 

The  student  finds  it  hard  to  refrain  from  bringing  to- 
gether considerations  of  velocity  and  those  of  pressure 
which  are  better  kept  separate.  Note  that  reduction  in 
velocity  means  a  widening  of  path  while  reduction 
pressure  means  resistance  overcome.  Observe  also  that 
velocity  may  increase  by  the  contraction  of  the  channel 
but  we  can  never  find  pressure  increasing  as  we  follow  the 
direction  of  the  flow  in  horizontal. vessels;  it  can  only 
decrease.  Extra  pressure  applied  at  the  starting  point 
may  indeed  increase  velocity,  but  the  acceleration  will 
be  in  all  parts  of  the  circuit  and  it  will  remain  true  as 
before,  that  the  highest  velocity  is  observed  where  the 
path  is  narrowest  and  the  lowest  where  it  has  the  greatest 
cross-section. 

The  Intermittent  and  the  Constant  Flow. — The  blood 
is  discharged  from  the  ventricles  in  successive  gushes 
with  pauses  between. .  Signs  of  an  intermittent  flow  are 
apparent  all  along  the  course  of  the  arteries  but  the  char- 
acter is  less  marked  as  the  small  branches  are  observed. 
In  the  capillaries  the  onward  movement  of  the  blood 
seems  steady  and  so  it  does,  generally  speaking,  in  the 
veins.  How  is  the  original  intermittency  overcome? 
The  answer  to  this  question  is  most  easily  discovered  by 
attending  to  the  artificial  devices  which  serve  a  similar 
purpose.  One  of  these  is  the  air  chamber  used  in  con- 
nection with  force  pumps. 


THE  COURSE  AND  PHYSICS  OF  THE  CIRCULATION       235 

This  feature  may  be  seen  in  a  fire  engine.  There  is  a 
large  dome  of  metal  which  contains  a  quantity  of  air  and 
is  in  communication  with  the  channel  which  conducts 
the  outflow  from  the  pumps.  The  air  inside  is  an  elastic 
body  or  cushion.  It  is  under  compression  to  an  extent 
determined  by  the  pressure  which  is  applied  to  the 
water  by  the  rapidly  working  pistons.  The  tendency  of 
the  confined  air  to  expand  maintains  the  discharge 
during  the  brief  interruptions  of  the  delivery  from  the 
pumps.  If  the  air  chamber  were  not  provided,  the 
stream  sent  into  the  hose  would  be  a  pulsatile  one. 

In  the  circulatory  system  there  is  no  large  and  localized 
organ  to  serve  the  purpose  of  an  air  chamber.  But  the 
elastic  property  is  represented  throughout  the  arterial 
tree.  The  larger  trunks,  in  particular,  swell  when  blood 
is  thrust  out  of  the  heart  and  exert  an  extra  pressure  upon 
their  contents  in  proportion  as  they  are  distended.  By 
their  contraction  when  the  discharge  from  the  heart 
ceases  they  continue  the  flow  in  their  smaller  branches 
and  thus  in  the  capillaries  and  veins. 

Economy  of  power  is  secured  by  the  conversion  of  the 
intermittent  to  a  constant  flow.  Much  more  force  would 
be  required  to  drive  the  same  volume  of  blood  through  a 
rigid  system  for  there  would  be  a  dead  loss  of  momentum 
after  every  heart-beat  and  all  the  blood  would  have  to 
be  started  afresh  from  a  state  of  arrest.  This  would 
severely  rack  the  pump.  Something  approaching  this 
condition  occurs  in  arterio-sclerosis.  The  hardened 
arteries  of  the  aged  do  not  yield  readily  to  the  inrush  of 
blood  from  the  ventricle.  Less  of  it  can  be  accommodated 
by  lateral  enlargement  and  more  has  to  be  pushed  straight 
ahead.  This  requires  the  development  of  greater  pres- 
sure than  would  be  called  for  in  a  more  elastic  system. 
Thus  the  heart  is  subjected  to  increasing  demands  when 
it  is  naturally  declining  in  reserve  strength.  It  is  one  of 
the  gravest  of  "vicious  cycles." 

Gravity  and  the  Circulation. — In  our  discussion  of  blood 
pressure  we  have  proceeded  thus  far  as  though  gravity 


236 


HUMAN    PHYSIOLOGY 


were  not  at  all  involved.  In  other  words  we  have  treated 
the  subject  as  though  the  heart  and  all  the  vessels  were 
in  the  same  horizontal  plane.  This  can  never  be  exactly 
the  case  and  it  is  very  far  from  the  condition  in  man  with 
his  erect  posture.  In  one  sense  gravity  does  not  in- 
fluence the  circulation:  there  is  always  as  much  blood 

_  Subtract 


50  mn, 
mercury 


H.L. 


FIG.  49. — To  suggest  how  the  weight  of  the  blood-columns  adds  to 
local  pressures  below  the  heart  and  subtracts  from  those  above  its 
level.  There  are  obscure  factors  which  somewhat  temper  the  extreme 
variations. 


falling  as  there  is  rising  in  the  system  and  we  have  to  do 
with  balanced  columns.  The  heart  does  not  permanently 
lift  any  weight  from  a  lower  to  a  higher  level.  But 
gravity  does  have  a  marked  influence  upon  local  pressure. 
All  blood  below  the  heart  presses  harder  upon  the 
vessels  than  it  would  at  the  heart's  own  level.  All  blood 


THE  COURSE  AND  PHYSICS  OF  THE  CIRCULATION      237 

above  the  heart  has  a  reduced  pressure  because  of  gravity. 
Suppose  that  a  man  has  been  lying  down  and  that  the 
average  pressure  in  the  root  of  his  aorta  is  125  mm.  of 
mercury.  He  rises  to  his  feet.  The  aortic  pressure  may 
change  somewhat  but  we  will  assume  that  it  does  not. 
A  moment  before  the  arterial  pressure  in  his  head  and  his 
feet  was  nearly  equal;  now  the  pressure  in  the  arteries  of 
his  scalp  is  reduced  by  about  50  mm.  and  that  in  his 
feet  is  increased  by  some  90  mm.  Still  the  heart  pressure 
is  ample  to  maintain  circulation  through  the  brain  and 
the  arteries  of  the  legs  are  not  capacious  enough  to 
accumulate  much  extra  blood. 

In  a  three-story  house  we  want  to  have  water  pressure 
enough  to  provide  for  a  flow  on  all  the  floors  at  once  but 
we  cannot  avoid  having  a  greater  pressure  in  the  base- 
ment than  in  the  attic.  On  the  whole  it  is  remarkable 
that  the  body  can  make  such  changes  of  position  and 
still  be  well  served  by  the  circulation.  When  there  is 
faintness,  the  influence  of  gravity  is  distressingly  ap- 
parent. The  usual  cause  of  faintness  is  an  insufficient 
blood-supply  to  the  brain  associated  with  a  failure  of  the 
normal  surplus  pressure  in  the  arteries  of  the  head.  It  is 
then  of  great  advantage  to  lie  down  and  bring  these  ves- 
sels into  a  position  where  they  are  no  longer  subjected 
to  a  handicap. 

A  most  striking  effect  of  gravity  is  to  create  an  ex- 
ception to  the  rule  that  venous  pressure  is  always  low. 
The  blood  in  the  veins  of  the  legs  and  feet,  when  the 
subject  is  standing,  exerts  a  very  considerable  pressure 
upon  the  confining  vessels.  In  the  feet  this  may  apr 
proach  100  mm.  of  mercury,  an  amount  which  is  almost 
of  the  general  arterial  order.  It  is  not  surprising  that 
varicose  veins  develop  in  the  lower  extremities. 

The  Portal  System. — According  to  our  description,  as 
far  as  we  have  gone,  the  blood  which  is  sent  out  from  the 
left  ventricle  is  forced  through  one  set  of  capillaries  and 
then  returns  to  the  right  side  of  the  heart  by  way  of  the 
systemic  veins.  We  must  now  indicate  an  important 


238  HUMAN    PHYSIOLOGY 

departure  from  the  type.  The  blood  which  is  supplied  to 
the  alimentary  canal  (below  the  esophagus)  is  not  re- 
turned directly  to  the  great  vein  which  passes  up  through 
the  abdominal  cavity  on  its  way  to  the  right  auricle. 
This  blood,  together  with  that  from  the  pancreas  and  the 
spleen,  is  gathered  into  a  vessel  called  the  portal  vein. 
This  is  unique  among  blood-vessels  of  any  size,  inasmuch 
as  it  is  made  by  the  union  of  small  branches — like  any 
vein — while  it  is  destined  to  subdivide  again  and  to 
supply  a  set  of  capillaries  as  though  it  were  an  artery. 
The  second  set  of  capillaries  is  in  the  liver. 

The  blood  which  has  passed  through  the  tissue  of  the 
liver  is  collected  by  a  number  of  short  veins  which  empty 
into  the  inferior  vena  cava.  This  circulatory  apparatus 
of  the  liver  is  spoken  of  as  the  portal  system  and  it  will  be 
seen  that  its  chief  peculiarity  consists  in  the  fact  that  the 
capillaries  are  " tandem"  to  those  of  the  digestive  organs, 
receiving  what  may  be  called  second-hand  blood.  The 
obvious  suggestion  is  that  the  liver  is  interposed  between 
the  alimentary  organs  and  the  rest  of  the  body  to  work 
over  the  absorbed  food  principles  before  admitting  them 
to  the  circulation  at  large.  The  liver  receives  a  secondary 
supply  of  arterial  blood  by  a  relatively  direct  channel 
from  the  aorta. 

The  Pulmonary  Circulation. — It  has  been  said  that 
the  right  ventricle  has  a  lighter  duty  than  the  left.  It 
has  to  pump  the  same  quantity  of  blood  in  a  unit  of  time 
but  the  mass  which  it  has  to  keep  moving  is  much 
smaller.  The  pressure  which  it  is  required  to  develop  is 
not  more  than  a  third  as  great  as  that  which  the  left 
ventricle  must  apply.  The  blood  is  sent  up  from  the 
right  ventricle  through  a  short,  wide  pulmonary  artery 
(carrying  venous  blood).  Under  the  arch  of  the  aorta 
the  pulmonary  artery  forks  to  supply  the  two  lungs. 
The  blood  comes  back  to  the  left  auricle  arterialized  and  is 
brought  in  by  four  pulmonary  veins,  two  from  each  side. 

The  Lymph  and  its  Movement. — The  term  lymph  is 
usually  given  to  all  the  fluid  outside  the  blood-vessels. 


THE  COURSE  AND  PHYSICS  OF  THE  CIRCULATION       239 

Some  of  this  fluid  is  in  spaces  between  the  organs  and  the 
body  walls.  Special  names  like  pleural  or  peritoneal 
fluid  may  be  used  in  such  cases.  Much  of  the  liquid  is  in 
microscopic  spaces  among  the  cells  of  the  various 
tissues.  Some  is  in  a  definite  set  of  vessels,  the  lymphatics. 
It  has  been  urged  that  we  should  reserve  the  word  lymph 
for  the  fluid  in  the  lymphatics,  call  that  in  the  micro- 
scopic interstices  tissue-fluid,  and  use  local  names  for  the 
contents  of  the  larger  cavities.  So  far  as  we  know  the 
composition  of  the  liquid  in  these  different  places  is  not 
far  from  uniform. 

Lymph  may  be  thought  of  as  fairly  represented  by 
the  contents  of.  a  blister.  The  exchanges  which  take 
place  between  the  blood-capillaries  and  the  tissues  are 
so  free  that  the  lymph  must  tend  to  have  the  same  com- 
position as  the  plasma.  It  has  been  roughly  described  as 
"  blood  minus  the  red  corpuscles/'  As  its  contact 
with  the  active  cells  is  so  close  we  should  expect  it  to 
contain  rather  less  of  the  substances  recognizable  as  foods 
and  rather  more  of  those  known  to  be  wastes  than  would 
be  found  in  the  blood.  This  difference  is  in  fact  realized. 
An  exception  may  be  noted  in  the  wall  of  the  small  intes- 
tine during  digestion  for  here  the  lymph  may  be  enriched 
with  foods  derived  from  the  canal. 

The  Lymphatics. — These  are  channels  through  which 
lymph  flows  slowly  from  all  parts  of  the  body  toward  two 
places  in  the  thorax  where  it  is  introduced  into  the 
systemic  veins.  We  ought  not  to  speak  of  a  lymphatic 
circulation  for  there  are  no  vessels,  corresponding  to 
arteries,  through  which  lymph  is  carried  outward  from 
the  chest.  The  movement  is  wholly  centripetal  and 
therefore  comparable  in  direction  with  the  flow  of  blood 
in  the  veins.  We  are  to  think  of  the  lymph  as  being 
formed  in  the  tissues  and  draining  away  quite  gradually 
to  lose  its  identity  at  last  in  the  venous  blood. 

There  has  been  much  doubt  as  to  how  the  finest 
lymphatics,  the  lymph-capillaries,  begin.  Some  have 
thought  of  them  as  originating  in  the  indefinite  micro- 


240 


HUMAN    PHYSIOLOGY 


scopic  spaces  of  the  tissues.  Others  have  been  led  to  the 
view  that  they  come  from  small  sacs  bounded  by  delicate 
epithelium.  But  even  if  the  latter  conception  is  correct 
we  may  continue  to  assume  that  matter  derived  from  the 

active  cells  can  find  its  way 
along  the  lymphatics.  The 
fluid  that  comes  from  an  or- 
gan through  the  lymphatics 
may  be  considered  an  over- 
flow of  plasma  from  the 
blood-capillaries,  modified 
by  the  give  and  take  of  the 
cells  which  it  has  bathed. 

The  great  majority  of  the 
lymphatics  contribute  to  one 
chief  channel.  This  is  the 
thoracic  duct.  It  is  formed 
below  the  diaphragm  by  the 
union  of  all  the  lymphatics 
from  the  legs,  the  pelvis,  the 
abdomen,  and  viscera.  The 
lymph  from  the  intestine  is 
at  times  laden  with  recently 
absorbed  fat.  The  duct  is 
continued  up  through  the 
thorax  in  front  of  the  ver- 
tebral column,  receiving 
branches  from  the  chest,  left 
arm,  and  the  left  side  of  the 
head.  It  empties  at  the  junc- 
tion of  two  great  veins,  one 
bringing  blood  from  the  left 
arm  and  one  coming  down 
At  the  corresponding  point 
on  the  right  there  is  a  rather  insignificant  lymphatic 
trunk  through  which  comes  the  lymph  from  the  right 
arm  and  shoulder  and  the  right  side  of  the  head. 

The  thoracic  duct  in  man  is  perhaps  as  large  as  a  goose- 


FIG.  50. — Lacteals  and  lym- 
phatics during  digestion.  (Re- 
drawn after  Collins.) 

the  left  side  of  the  neck. 


THE  COURSE  AND  PHYSICS  OF  THE  CIRCULATION       241 

quill.  If  a  vein  of  this  size  were  cut  it  would  yield  a 
large  outflow  of  blood  in  a  short  time.  By  contrast,  the 
flow  of  lymph  from  the  cut  thoracic  duct  is  a  mere  dribble. 
The  question  arises  why  the  lymph  flows  at  all.  The 
fundamental  fact  is  that  it  is  formed  under  some  pressure 
in  the  tissues  while  an  outlet  into  the  great  veins  is  pro- 
vided at  a  place  where  there  is  practically  no  opposing 
pressure.  It  is  also  permissible  to  say  that  the  continu- 
ous formation  of  new  lymph  is  bound  to  crowd  along  in 
the  lymphatics  the  fluid  that  entered  them  previously. 

An  important  factor  contributing  to  the  movement  of 
the  lymph  is  muscular  contraction.  The  larger  lym- 
phatics are  provided  with  simple  valves  which  allow  an 
advance  toward  the  thorax  but  forbid  a  return.  When 
the  skeletal  muscles  throw  pressure  upon  the  lymphatics 
the  lymph  is  made  to  slip  along  toward  its  distant  outlet 
and  when  the  pressure  is  removed  the  refilling  of  the 
emptied  vessels  can  occur  only  from  their  small  branches. 
There  are  valves  in  the  veins  of  the  limbs  which  similarly 
promote  the  venous  circulation  during  exercise. 

Lymph  Nodes. — Lymph  which  is  moving  from  an 
extremity  toward  the  thorax  must  pass  somewhere  on  its 
way  at  least  one  of  the  structures  known  as  lymphatic 
glands  or,  better,  as  lymph  nodes.  These  are  small 
kernels  of  fairly  dense  tissue  interposed  in  the  course  of 
the  lymphatics,  particularly  in  the  abdomen,  the  neck, 
armpits,  and  groins.  Microscopic  study  shows  that 
they  must  act  mechanically  as  filters,  the  lymph  working 
through  their  narrow  interstices.  Certain  cells  from 
these  nodes  may  become  detached  and  pass  on  with  the 
lymph  to  the  blood,  furnishing  one  type  of  colorless 
corpuscle.  The  lymph  nodes  probably  constitute  de- 
fenses against  the  march  of  infection  along  lymphatic 
pathways. 

The  Circulation  before  Birth. — The  arrangement  of 
the  circulatory  system  in  the  unborn  child  is  so  different 
from  the  permanent  condition  that  the  changes  occurring 
at  birth  seem  almost  miraculous.  The  critical  character 

16 


242  HUMAN    PHYSIOLOGY 

of  the  emergency  and  the  success  with  which  it  is  usu- 
ally met  make  a  story  which  is  too  fascinating  to  be 
slighted. 

The  lungs  of  the  embryo  are  dense  and  compressed. 
No  air  has  ever  entered  them.  Their  blood-vessels  as 
well  as  their  air  passages  are  nearly  closed.  But  little 
blood  makes  its  way  from  the  right  ventricle  to  the  left 
auricle  through  the  pulmonary  circuit.  A  temporary 
provision  is  made  to  enable  the  right  ventricle  to  send 
out  blood  in  spite  of  the  obstructed  state  of  the  vessels 
in  the  lungs.  The  device  referred  to  is  the  ductus 
arteriosus,  a  by-pass  from  the  pulmonary  artery  to  the 
aorta  close  by.  The  blood  expelled  by  the  right  ventricle 
slips  through  to  join  that  pumped  by  the  left  and  the 
result  is  that  both  ventricles  are  united  to  supply  the 
arteries  of  the  child. 

As  little  blood  passes  through  the  lungs  there  can  be 
little  to  enter  the  left  auricle  through  the  four  pulmonary 
veins.  But  there  is  an  opening  through  the  middle 
partition  between  the  two  auricles  called  the  foramen 
ovale.  Through  this  the  left  auricle  is  supplied  by  an 
overflow  from  the  right.  We  have  seen  that  both 
ventricles  deliver  to  the  arteries  and  now  we  find  that 
both  auricles  share  the  blood  returning  from  the  veins. 
We  have  next  to  show  how  the  lack  of  lungs  is  supplied. 
The  substitute  is  the  organ  called  the  placenta. 

The  placenta  is  a  disc  of  tissue  rich  in  blood-vessels 
which  is  in  close  contact  with  the  lining  of  the  uterus. 
A  long  cord  (the  umbilical  cord)  unites  the  placenta 
with  the  navel  of  the  child.  The  cord  serves  chiefly  to 
convey  blood  to  and  from  the  placenta.  It  contains 
two  branches  of  the  arterial  system  of  the  embryo  which 
are  derived  from  the  permanent  vessels  in  the  pelvis. 
The  blood  which  is  diverted  through  these  vessels  is  sent 
through  minute  spaces  of  the  placenta  where  it  is  sepa- 
rated only  by  the  thinnest  of  walls  from  the  maternal 
blood  in  the  wall  of  the  uterus.  A  single,  so-called  vein 
comes  back  in  the  cord  and  delivers  the  blood  partly  to 


THE  COURSE  AND  PHYSICS  OF  THE  CIRCULATION       243 

the  portal  vein  and  partly  to  the  inferior  vena  cava  of 
the  embryo. 


FIG.  51. — A  conventional  diagram  of  the  circulation  before  birth 
(compare  with  Fig.  45).  Spatial  relations  are  ignored.  It  will  be 
seen  that  both  ventricles  pump  into  the  arteries,  that  a  diversion  of 
blood  takes  place  to  the  placenta  (at  the  right  of  the  diagram)  and  that 
the  returning  blood  may  enter  the  portal  vein  or  the  general  venous 
system.  What  is  less  clear  is  the  filling  of  the  left  auricle  from  the  right, 
indicated  by  a  dotted  arrow. 


The  usual  distinctions  of  arterial  and  venous  blood 
cannot  be  made  in  this  case.  The  blood  is  nowhere  up 
to  the  arterial  standard  of  the  mother.  The  best  of  it 


244  HUMAN    PHYSIOLOGY 

is  that  which  has  been  improved  in  the  placenta  by 
effecting  exchanges  with  the  blood  in  the  uterine  vessels. 
This  is  introduced  into  the  veins  of  the  child.  The 
stream  is  sufficiently  abundant  to  keep  the  average 
composition  of  the  blood  suitable  for  nutrition.  The 
placenta  is  more  than  a  lung;  it  is  the  seat  of  absorption 
of  organic  food  and  serves  for  the  unloading  of  wastes. 
It  takes  the  place,  for  the  time,  of  the  intestine  and  the 
kidneys. 

At  birth,  the  placental  circulation  is  interrupted. 
The  child  is  cast  upon  its  own  resources  and  it  must  begin 
to  use  its  lungs  or  perish  from  suffocation.  The  stimulus 
to  the  medulla  to  initiate  breathing  may  be  due  to  the 
rising  percentage  of  carbon  dioxid  in  the  blood  but  it  is 
likely  to  be  reinforced  by  the  effect  of  strange  contacts 
and  an  unwonted  temperature  at  the  surface  of  the  body. 
When  the  first  breath  is  taken  the  larger  respiratory 
passages  are  full  of  liquid.  This  retreats  from  the  nostrils 
as  the  air  enters  and  the  compressed  system  is  opened 
out  much  as  one  expands  a  Christmas  bell  of  tissue 
paper.  The  same  movement  that  dilates  the  air  spaces, 
rounds  the  flattened  capillaries  of  the  lungs  and  im- 
mediately a  large  share  of  the  blood  pumped  out  by  the 
right  ventricle  takes  its  course  through  them. 

The  possibility  of  some  crossing  over  between  the  two 
sides  of  the  heart  remains  for  a  time  but  the  ductus 
arteriosus  and  the  foramen  ovale  are  quite  rapidly  closed. 
In  a  few  days,  in  normal  cases,  no  blood  can  escape 
going  through  the  lungs  after  it  has  journeyed  through 
the  systemic  circulation.  Occasionally  there  is  an  un- 
fortunate failure  to  close  the  embryonic  by-passes  and 
so  much  venous  blood  enters  the  arterial  system  that  a 
"  blue  baby  "  is  the  result.  In  such  instances  the  chances 
of  survival  are  precarious  and  there  is  more  or  less  marked 
malnutrition. 


CHAPTER  XVIII 
THE  HEART 


The  general  function  of  the  heart  should  be  clear  in  the 
light  of  what  has  gone  before.  It  is  the  double  force 
pump  at  the  center  of  the  circulation.  It  is  conveniently 
thought  of  as  being  about  the  size  of  the  individual's 
fist  and  it  is  placed  in  the  chest  between  the  lungs. 


FIG.  52. — Suggesting   the   position   of   the   heart  in   the   thorax. 

More  than  half  of  it  is  to  the  left  of  the  mid-plane.  It  is 
an  irregular  cone  with  its  apex  directed  downward,  for- 
ward, and  to  the  left.  This  apex  comes  close  to  the  wall 
of  the  chest  between  the  fifth  and  the  sixth  ribs,  a  fact 
easily  remembered  in  connection  with  II  Samuel  III,  27. 
The  heart  is  enclosed  by  a  sac  called  the  pericardium 
which  is  really  a  continuation  of  its  outer  layer,  reflected 

245 


246 


HUMAN    PHYSIOLOGY 


from  the  roots  of  the  blood-vessels.  In  the  pericardium 
is  a  small  quantity  of  fluid.  What  we  call  the  right  side 
of  the  heart  is  thrown  to  the  front  by  a  spiral  twist  of  the 
organ  so  we  see  chiefly  this  ventricle  when  the  heart  is 
exposed  from  in  front.  The  left  ventricle  is  thrown 
behind  but  the  apex  belongs  to  it;  it  may  be  said  to  be 

longer  than  the  right.  If  the 
ventricles  are  cut  across  it  is 
at  once  apparent  that  the  left 
ventricle  has  the  heavier  walls. 
The  cavity  of  the  right  ven- 
tricle in  such  a  section  is  cres- 
centic  and  it  seems  an  appen- 
dage upon  the  left.  (Fig.  53.) 
The  heart  is  supported  by 
the  clustered  vessels  and  ad- 
jacent structures.  The  ar- 
teries and  veins  may  be  re- 
called: there  is  the  aorta, 
arching  up  from  the  left  ven- 
tricle, the  short  pulmonary 
artery  rising  from  the  right 
ventricle  and  forking  under 
the  arch  of  the  aorta,  the  two 
venae  cavae  connecting  with 
the  right  auricle,  and  the  four 
pulmonary  veins  entering  the 
left  auricle.  There  is  a  well- 
marked  groove  between  the 
auricles  above  and  the  ventri- 
cles below.  The  auricles  have  uneven  contours  while  the 
ventricles  are  smooth. 

The  cavities  of  the  heart  are  lined  by  an  epithelium 
like  that  of  the  capillaries.  Between  this  and  the 
pericardium  the  essential  tissue  is  muscle  of  the  unique 
order  which  we  call  cardiac.  The  cells  are  short  cylinders 
united  end  to  end  in  bundles.  The  strands  which  result 
have  a  most  intricate  arrangement;  in  general  it  may  be 


FIG.  53. — A  general  view  of 
the  heart,  the  ventricles  being 
cut  across  to  show  the  crescen- 
tic  form  of  the  right,  appearing 
like  an  appendage  upon  the  left. 


THE    HEART 


247 


said  that  they  pass  spirally  around  all  the  chambers  of 
the  heart.  By  their  united  contraction  the  cavities  are 
diminished  and  may  be  well  nigh  obliterated,  although 
we  cannot  assume  that  this  happens  at  each  heart-beat. 
We  are  not  to  think  that  the  solid  tissues  of  the  heart  lose 
volume  when  the  act  of  contraction  is  performed  but  the 
organ  externally  becomes  smaller  just  in  proportion  as 
the  blood  is  pressed  out  of 
its  ventricles. 

The  Heart  Valves. — As 
the  chambers  of  the  heart 
are  rhythmically  widened 
and  narrowed,  the  blood 
is  driven  in  a  fixed  direc- 
tion. This  could  not  be 
the  case  if  it  were  not  for 
the  two  sets  of  valves  with 
which  the  organ  is  pro- 
vided. If  we  speak  of 
those  on  the  left  side  the 
description  will  answer  ?IG  54._For  the  sake  of  clear_ 

approximately     for     those      ness  the  two  halves  of  the  heart  are 

On   the   right.       We  find  a      separated  by  an  interval   (r.a.)  and 

,        ,  , ,  .   ,         (l-a.)  are  the  right  and  left  auricles; 

valve  between  the  auricle     (r.l).)  and  (i.v.)  are  the  right  and  left 

and  the  ventricle  and  an-      ventricles;  (1)  is  the  tricuspid,   (3) 

other  valve  or  set  of  valves  tales'™1' (2)  "^  (4)  ^  semihmar 
in  the  root  of  the  great 

artery.  The  valves  are  membranous  flaps  with  no 
power  to  move  save  as  the  currents  of  blood  actuate  them. 
On  the  left  side  the  valve  guarding  the  opening  from  the 
auricle  to  the  ventricle  is  called  the  mitral  valve.  Its 
fellow  on  the  right  is  the  tricuspid. 

Students  often  find  it  hard  to  remember  which  is  on 
the  right  and  which  on  the  left.  Huxley's  mnemonic 
device  may  be  quoted  (without  endorsing  the  statement) : 
"a  mitre  is  a  bishop's  crown  and  a  bishop  would  never  be 
found  on  the  right  side."  The  mitral  valve  consists  of 
two  thin  and  transparent  but  very  tough  sheets  of  tissue 


248 


HUMAN    PHYSIOLOGY 


which  are  directed  well  down  into  the  ventricle  from  the 
edges  of  the  orifice  leading  from  the  auricle.  They  are 
never  far  apart  but  blood  can  pass  down  between  them 
freely,  while  the  first  sign  of  a  reflux  will  bring  them 
together.  When  closure  has  been  accomplished,  added 
pressure  in  the  ventricle  will  only  press  the  two  flaps 
more  firmly  one  against  the  other.  The  lower  edges  of 
these  flaps  are  limited  in  their  movement  by  a  number 
of  fine  "  tendinous  cords "  leading  to  the  wall  of  the 
ventricle. 


FIG.  546. — The  principle  of  the  semilunar  valves.  At  (1)  the  root  of 
the  aorta  is  laid  open  and  spread  to  show  the  three  pockets.  At  (2)  we 
look  down  into  the  vessel  when  the  passage  is  open.  At  (3)  the  flaps 
lie  in  contact  closing  the  channel. 

The  valves  in  the  root  of  the  aorta  are  called  semilunar. 
The  same  name  is  applied  to  those  in  the  beginning  of 
the  pulmonary  artery.  In  either  case  dissection  shows 
three  pockets  on  the  inner  surface  of  the  vessel.  Their 
free  borders  are  directed  away  from  the  heart.  The  pass- 
ing of  the  blood-stream  in  this  the  normal  direction  will 
lay  the  flaps  back  against  the  wall  of  the  artery  and 
there  will  be  a  free  channel  among  them.  A  reversal  of 
the  flow  will  fill  the  pockets  and  throw  them  together, 
closing  the  passage.  Each  flap  has,  at  the  middle  point 


THE    HEART  249 

of  its  edge,  a  small  thorn-like  projection,  and  when  the 
valves  are  closed  these  cartilaginous  processes  are  locked 
in  the  center  of  the  vessel  as  shown  in  the  diagram. 

The  Heart-beat. — By  a  heart-beat  we  mean  a  coordi- 
nated contraction  of  the  cardiac  muscle  resulting  in  the 
expulsion  of  blood  from  both  ventricles.  The  heart  of  a 
man  at  rest  may  beat  seventy-two  times  a  minute.  Some 
individuals  have  a  lower  and  some  a  higher  average. 
The  resting  rate  may  be  more  than  doubled  by  violent 
exercise.  After  each  beat  there  is  an  interval  during 
which  the  heart  is  passive.  The  term  heart  cycle  is  used 
to  cover  the  combination  of  the  active  and  the  passive 
phases,  the  round  of  events  connected  with  a  single 
beat  of  the  heart  and  including  the  preparation  for  a 
succeeding  beat.  Two  old  words  have  survived  in  the 
description  of  the  heart  action  which  are  almost  obsolete 
elsewhere  in  physiology:  the  terms  systole  and  diastole. 
The  former  means  contraction  and  the  latter  relaxation. 

A  description  of  the  sequence  of  events  making  up  a 
heart  cycle  may  be  begun  with  any  phase  that  we  choose 
inasmuch  as  we  have  to  do  with  recurring  conditions. 
It  will  be  convenient  to  start  with  the  period  of  general 
relaxation  which,  as  a  matter  of  fact,  lasts  about  half 
the  whole  time.  If  we  assume  that  the  heart  is  beating 
seventy-five  times  a  minute  we  have  to  assign  to  one  of  its 
cycles  0.8  second.  During  about  0.4  second  in  each 
cycle  the  heart  is  not  manifesting  any  active  contraction. 
No  blood  is  being  pressed  out  of  it.  On  the  contrary, 
all  its  cavities  are  increasing  in  capacity  and  receiving 
the  inflow  from  the  veins. 

While  this  passive  condition  continues,  the  mitral  and 
tricuspid  valves  hang  apart  and  we  do  well  to  think  of  the 
auricle  and  ventricle  of  either  side  as  forming,  for  the 
time,  a  single  chamber.  The  incoming  blood  may 
gather  in  the  widening  auricle  or  pass  on  to  the  ventricle. 
If  this  is  clearly  grasped  the  student  will  be  saved  from 
a  common  mistake:  that  of  thinking  that  the  blood  all 
accumulates  in  the  auricle  before  any  is  forwarded  to 


250 


HUMAN    PHYSIOLOGY 


the  ventricle.     At  the  close  of  the  diastolic  period  the 
ventricles  as  well  as  the  auricles  are  moderately  distended. 


TIM. 


FIG.  55. — The  heart  cycle.  The  circumference  of  the  circle  is 
divided  into  eight  parts,  each  standing  for  0.1  second.  At  (6)  the  heart 
is  in  a  passive  state  and  swelling  as  it  receives  blood.  Between  (8) 
and  (1)  the  auricle  thrusts  on  some  blood  to  the  ventricle  which  reaches 
its  maximum  capacity  at  (1)  (max.).  Between  (1)  and  (4)  the  ven- 
tricle discharges  to  the  arteries  and  is  reduced  to  its  minimum  capacity 
at  (4).  While  the  ventricle  is  emptying,  the  auricle  is  storing  blood  and 
is  large  at  (4).  The  diagram  applies  to  either  side  of  the  heart.  A.v.v. 
stands  for  auriculo-ventricular  valves,  S'l.v.  for  semilunar  valves. 

The  first  sign  of  contraction  in  the  heart  is  noted  in 
the  auricles.  Careful  observation  has  shown  that  there 


THE    HEART  251 

is  a  particular  spot  in  the  right  auricle  from  which  the 
process  radiates.  For  the  purposes  of  elementary  de- 
scription we  may  say  that  the  two  auricles  contract 
simultaneously.  The  systole  of  the  auricles  is  brief  and 
not  at  all '  forcible.  Its  effect  is  to  transfer  a  small 
quantity  of  blood  to  the  ventricles  which  are  thus  put 
upon  a  stretch  beyond  their  previous  state.  As  the 
auricles  relax  after  their  momentary  contraction,  the 
blood  recoiling  from  the  tense  walls  of  the  ventricles 
brings  together  the  flaps  of  the  mitral  and  tricuspid 
valves. 

It  is  to  be  noted  that  there  are  no  valves  to  guard  the 
openings  by  which  'the  veins  empty  into  the  auricles. 
Hence  it  may  be  asked  why  the  contraction  of  the  auricles 
should  not  turn  back  the  blood  in  these  vessels.  As  a 
matter  of  fact  there  is  something  of  a  check  imposed  on 
the  flow  in  the  veins  of  the  thorax  and  neck  when  the 
auricles  are  contracting.  But  it  is  only  slight  because 
the  time  of  the  backward  thrust  is  so  short  and  the 
momentum  of  the  blood  column  so  great.  It  is  probable 
that  the  systole  of  the  auricles  has  a  progressive  or 
peristaltic  character,  tending  to  push  the  blood  toward 
the  ventricles  rather  than  in  the  reverse  direction. 

When  the  mitral  and  tricuspid  valves  have  been 
closed  there  is  a  brief  pause.  It  is  an  interval  during 
which  the  excitation  is  being  transmitted  from  the 
tissue  of  the  auricles  to  that  of  the  ventricles.  There  is 
a  zone  between  the  upper  and  the  lower  chambers  of  the 
heart  in  which  there  is  little  or  no  muscle.  But  a  peculiar 
conducting  strand,  the  bundle  of  His,  bridges  this 
region  and  maintains  the  physiologic  continuity  of  auricle 
and  ventricle.  Through  the  bundle  of  His  the  muscle 
of  the  ventricles  is  presently  stimulated  and  the  systole 
of  these  chambers  follows.  This  is  the  essential  feature 
of  the  heart-beat. 

The  two  ventricles  are  contracted  at  the  same  time. 
Their  systole  occupies  about  0.3  second  under  aver- 
age conditions.  The  pressure  developed  quickly  rises 


252  HUMAN    PHYSIOLOGY 

to  a  maximum  and  as  soon  as  it  exceeds  the  back 
pressure  in  the  arteries  it  forces  apart  the  semilunar 
valves.  A  rapid  emission  of  blood  follows.  When  the 
systole  of  the  ventricle  wanes  it  does  so  with  some 
suddenness;  the  internal  pressure  falls  below  that  in  the 
arteries  and  the  incipient  reflux  of  blood  closes  the 
semilunar  valves  sharply.  Twice  in  the  course  of  the 
heart  cycle  the  ventricles  are  closed  cavities:  first,  just 
after  the  auricular  contraction  when  their  content  of 
blood  is  greatest,  and  second,  when  their  own  systole 
ends  and  their  volume  is  least. 

The  amount  of  blood  thrown  from  either  ventricle 
during  its  contraction  has  been  estimated  all  the  way 
from  2  to  6  ounces.  The  highest  figure  is  based  on  the 
assumption  that  the  ventricle  is  considerably  distended 
and  then  practically  obliterated.  This  cannot  be  as- 
sumed and  a  medium  quantity  of  4  ounces  seems  more 
probable.  Pumping  at  such  a  rate,  one  ventricle 
would  put  out  about  all  the  blood  in  the  body  in  the 
course  of  40  beats  of  the  heart.  This  justifies  the  state- 
ment that  the  average  corpuscle  is  sent  from  a  given 
point — say  the  left  ventricle — around  the  systemic  and 
the  pulmonary  circuits  and  back  to  the  place  of  beginning 
about  twice  in  a  minute. 

At  the  close  of  the  ventricular  systole  a  moderate 
reduction  of  the  pressure  in  the  ventricle  will  lead  to  the 
closure  of  the  semilunar  valves.  The  pressure  must 
fall  much  lower  to  permit  the  mitral  and  tricuspid  valves 
to  open.  But  the  relaxation  is  swift  and  this  event 
follows  within  a  few  hundredths  of  a  second.  The 
ventricle  ceases  to  exert  any  pressure  upon  its  contents 
and  for  a  brief  interval  its  walls  are  actually  drawing 
apart  with  a  slight  suction  effect.  The  cause  of  this 
outward  movement  will  be  apparent  when  we  have 
considered  the  mechanical  conditions  in  the  chest  as  we 
shall  do  when  taking  up  the  subject  of  respiration. 

While  the  mitral  and  tricuspid  valves  are  closed,  the 
blood  brought  to  the  heart  by  the  veins  must  be  ac- 


THE    HEART  253 

commodated  in  the  auricles.  This  is  in  fact  one  of  the 
distinct  services  performed  by  these  chambers.  When 
the  above-mentioned  valves  open  the  auricles  are  very 
full  and  a  rapid  redistribution  of  blood  between  the 
auricles  and  the  ventricles  must  take  place  in  the  first 
part  of  the  ventricular  diastole,  (n)  in  Fig.  55.  There 
are  therefore  two  times  in  the  cycle  when  the  blood 
moves  at  an  accelerated  pace  into  the  ventricles:  first 
when  the  valves  open  to  admit  it  from  the  overfilled 
auricles  and;  second,  when  the  active  contraction  of  the 
auricles  thrusts  on  the  latest  portion  of  the  ventricular 
charge. 

The  Heart  Sounds. — Each  heart-beat  is  attended  by 
the  production  of  two  sounds  which  have  long  been 
closely  studied  because  of  their  relation  to  the  recog- 
nition of  disease.  To  the  trained  ear  of  the  physician 
their  subtle  variations  are  most  significant.  In  a  book 
like  this  we  can  give  them  only  a  little  space.  The  first 
is  described  as  a  prolonged  low-pitched  and  booming 
note;  the  second  is  higher  in  pitch  and  has  a  staccato 
character.  It  may  be  called  a  click.  It  can  be  shown 
that  the  first  sound  occurs  during  the  systole  of  the 
ventricles  and  the  second  comes  at  the  close  of  this 
phase. 

It  is  believed  that  the  first  sound,  long  represented 
by  the  syllable  "lubb,"  has  a  somewhat  complex  origin. 
We  have,  entering  into  it,  a  muttering  from  the  tense 
walls  of  the  ventricles,  the  rush  of  blood  through  the 
outlets,  and  perhaps  the  humming  of  the  taut  valves 
which  are  preventing  a  return  to  the  auricles.  If  either 
the  mitral  or  the  tricuspid  valve  is  imperfect  and  there 
is  any  considerable  reflux  from  one  of  the  ventricles 
during  their  systole  the  first  sound  will  be  altered. 

The  second  sound  is  usually  reproduced  by  the  syllable 
"diip."  Its  origin  is  quite  clear:  it  is  the  snap  of  the 
semilunar  valves  as  they  close  at  the  beginning  of  the 
ventricular  diastole.  It  can  be  reproduced  in  a  lifeless 
preparation  by  injecting  fluid  backward  into  the  artery 


254  HUMAN    PHYSIOLOGY 

so  as  to  throw  the  valves  into  contact.  Naturally,  when 
there  is  incompetency  on  the  part  of  these  valves  and 
some  blood  reenters  one  of  the  ventricles  during  its  re- 
laxation the  second  sound  will  be  changed.  It  will  lose 
its  crispness  and  be  prolonged  as  a  murmur. 

In  any  valvular  disease  of  the  heart  the  obvious  result 
is  that  more  than  the  normal  amount  of  work  must  be 
done  to  maintain  the  full  volume  of  the  circulation. 
More  or  less  of  the  blood  slips  back  and  has  to  be  pumped 
twice  instead  of  once.  The  extra  labor  is  the  same  in 
principle  as  that  which  must  be  performed  with  any 
leaky  pump  when  it  is  required  to  deliver  a  certain 
amount  of  water.  A  heart  which  has  to  contend  with 
such  a  condition  is  exercised  and  trained  like  any  other 
muscle  and  it  may  act  well  for  many  years.  Its  walls 
may  be  much  thickened  •  for  their  heavy  duty.  But  it 
will  be  seen  that  such  a  heart  can  scarcely  be  expected  to 
have  as  large  a  reserve  power  as  the  normal  organ.  It 
is  habitually  working  nearer  to  its  limit. 

Some  Properties  of  Cardiac  Muscle. — In  the  foregoing 
description  of  the  events  of  a  heart  cycle  but  little  was 
said  of  the  contractile  tissue  at  work.  We  must  now 
make  some  comparison  between  this  and  other  forms 
of  muscle.  We  shall  find  that  in  many  respects  it  is 
intermediate  between  the  smooth  and  the  skeletal  types. 
This  is  true,  first  of  all,  of  its  microscopic  organization. 
Its  cells1  are  short  and  have  each  a  single  nucleus,  agree- 
ing in  this  with  those  of  smooth  muscle.  But,  on  the 
other  hand,  they  are  transversely  striated  somewhat  like 
the  skeletal  fibers.  In  speed  9f  movement  they  are 
also  intermediate;  they  can  work  more  rapidly  than  any 
smooth  muscle  but  they  do  not  equal  the  skeletal  in  its 
capacity  to  contract  and  relax  quickly. 

The  most  remarkable  power  of  cardiac  muscle  is  its 
automaticity.  We  have  seen  that  skeletal  muscle  seldom 

1  The  lines  of  demarcation  between  these  cells  are  ill  denned  and  in 
many  respects  they  seem  merged  as  a  continuous  structure — a  syn- 
cytium. 


THE    HEART 


255 


contracts  unless  in  response  to  the  arrival  of  impulses  by 
way  of  the  motor  nerves.  Smooth  muscle  shows  an 
automatic  tendency  and  this  rhythmic  property  is  most 
highly  developed  in  the  heart.  A  summary  demonstra- 
tion is  obtained  when  the  heart  of  a  frog  is  removed  from 
the  body  of  the  animal.  The  organ  continues  to  beat. 
This  makes  it  plain  that  it  does  not  depend  on  the  central 
nervous  system  for  the  initiation  of  each  beat  and  we  must 
remember  that  there  is  just  this  difference  between  the 
heart  action  and  the  breathing: 
the  heart  beats  of  itself  while  the 
breathing  muscles  are  thrown 
into  contraction  by  the  brain. 

The  heart  of  a  mammal  taken 
from  the  body  declines  rapidly, 
though  its  automatic  nature  is 
evidenced  by  the  execution  of  at 
least  a  few  contractions.  Ex- 
periment has  shown  that  the 
reason  the  heart  of  a  frog  beats 
longer  than  that  of  a  cat  after 
its  isolation  is  chiefly  that  the 
latter  has  a  greater  need  of  oxygen.  If  this  want  is 
supplied  by  an  artificial  circulation  of  blood,  or  other- 
wise, the  cat's  heart  will  survive  for  a  long  time.  It  is  as 
truly  automatic  as  the  cold-blooded  heart. 

When  the  different  regions  of  the  heart  are  compared 
it  is  found  that  the  degree  of  automatic  power  is  not 
everywhere  the  same.  The  rhythmic  tendency  is  more 
marked  in  the  auricles  than  in  the  ventricles.  The 
student  must  not  be  confused  by  the  fact  that  the  part 
of  the  heart  which  is  physically  weaker  has  a  stronger 
inclination  to  activity.  The  ventricles  are  far  more 
massive  than  the  auricles  and  do  a  heavier  work  but  their 
rhythm  is  believed  to  be  dictated  by  these  less  robust 
divisions  of  the  heart.  It  will  be  desirable  to  cite  certain 
experiments  bearing  on  this  matter. 


FIG.    56. — Cardiac     muscle 
cells  (or  syncytium). 


256  HUMAN    PHYSIOLOGY 

Heart-block. — If  a  thread  is  tied  around  the  heart 
of  a  frog  or  a  turtle  between  the  auricles  and  the  single 
ventricle,  pressure  may  be  applied  in  the  groove  until 
the  tissue  at  that  level  is  seriously  injured.  It  is  then 
usually  observed  that  the  auricles  go  on  beating  as  be- 
fore but  that  the  ventricle  is  arrested  and  remains  re- 
laxed. The  conclusion  might  be  drawn  that  the  ventricle 
beats  in  the  normal  heart  because  of  rhythmic  stimula- 
tion passed  on  to  it  from  the  auricles.  This  appears 
to  be  the  case.  Another  conclusion  might  suggest  its 
self:  namely,  that  the  ventricle  has  no  automaticity. 
This  is  not  justified  when  all  the  facts  are  taken  into 
consideration. 

The  ventricle  will  beat  without  the  auricles  if  it  is 
bathed  within  by  blood  or  certain  solutions  which  majr 
be  substituted  therefor.  Its  rate  when  beating  inde- 
pendently is  lower  than  that  of  the  auricles.  Reference 
has  been  made  to  the  bundle  of  His  which  conducts 
the  excitation  from  the  auricles  to  the  ventricles  in  the 
hearts  of  the  higher  animals.  This  bundle  may  be  in- 
terrupted by  a  skillful  operation,  a  hook  being  inserted 
so  as  to  pass  behind  it  and  then  used  to  bring  pressure 
upon  it.  When  the  bundle  of  His  has  been  disorgan- 
ized, the  auricles  beat  as  before  while  the  ventricles 
adopt  a  new  and  slower  rate.  A  condition  has  been 
created  which  is  known  as  heart-block.  The  experi- 
ment confirms  the  inference  made  with  the  frog's  heart 
that  the  auricles  usually  impress  upon  the  ventricle 
their  own  rhythm  but  we  see  at  the  same  time  what 
could  not  be  made  out  in  the  other  case,  that  the  ven- 
tricular tissue  is  automatic  when  the  circumstances  are 
favorable. 

Heart-block  such  as  can  be  produced  by  injuring  the 
bundle  of  His  is  supposed  to  have  many  points  of 
similarity  with  a  malady  known  as  Stokes-Adams  dis- 
ease. The  victim  of  this  disorder  suffers  at  times 
from  inadequate  circulation  and  examination  shows  that 
the  pulse  at  the  wrist  falls  during  the  seizures  to  per- 


THE    HEART  257 

haps  30  or  40  beats  per  minute.  At  the  same  moment 
when  the  low  count  is  obtained  at  the  wrist  the  jugular 
vein  may  be  seen  to  pulsate  75  times  a  minute,  a  normal 
frequency.  Now  the  throb  of  any  artery  corresponds 
with  the  beat  of  the  left  ventricle  while  the  pulsation  of 
a  vein  is  in  the  rhythm  of  the  right  auricle.  Therefore 
it  is  clear  that  in  a  case  of  Stokes-Adams  disease  there 
is  a  failure  on  the  part  of  the  ventricles  to  adopt  the 
pace  of  the  auricles.  In  other  words  this  is  an  instance 
of  heart-block. 

A  somewhat  whimsical  but  useful  comparison  may  be 
made  between  the  auricle  and  ventricle  on  the  one  hand 
and  a  certain  type  of  married  couple  on  the  other. 
We  often  see  a  wife  of  slight  physique  but  great  vivacity 
and  ambition  who  compels  a  stalwart  but  naturally 
phlegmatic  husband  to  maintain  the  pace  which  she  sets 
for  him.  If  we  pursue  the  analogy  we  shall  see  in 
Stokes-Adams  disease  the  suggestion  of  a  declaration  of 
independence  on  the  part  of  the  husband,  an  assertion  of 
his  right  to  take  his  own  time  on  the  way. 

It  is  known,  as  the  result  of  a  great  body  of  research, 
that  the  automatic  property  of  cardiac  muscle  is  not 
exhibited  unless  the  solution  bathing  the  tissue  contains 
certain  mineral  constituents  in  definite  proportions. 
This  is  true  whether  we  speak  of  blood  or  of  some  more 
simple  substitute.  The  salts  most  certainly  necessary 
are  sodium,  calcium,  and  potassium  compounds.  The 
subject  is  interesting  but  the  details  must  be  sought  in 
larger  books. 

Another  subject  which  we  can  do  no  more  than  indi- 
cate is  the  question  whether  the  automaticity  of  heart 
tissue  is  a  property  of  the  contractile  cells  or  of  nervous 
elements  associated  with  them.  Exceedingly  small 
fragments  of  the  organ  may  carry  on  rhythmic  activity, 
but  it  is  difficult  to  be  sure  that  these  bits  do  not  contain 
nervous  as  well  as  muscular  units.  Most  physiologists 
are  inclined  to  believe  that  the  automatic  tendency  is 
inherent  in  the  cells  of  cardiac  muscle,  but  the  view  that 

17 


258  HUMAN    PHYSIOLOGY 

it  resides  in  intermingled  nerve-cells  continues  to  have 
able  defenders. 

The  All  or  None  Principle. — The  bloodless  ventricle, 
or  a  strip  cut  from  it,  may  be  used  to  study  the  response 
of  the  local  muscle  to  stimuli  just  as  is  done  so  often  with 
the  gastrocnemius.  (It  will  be  recalled  that  the  ventricle 
without  blood  is  not  likely  to  make  spontaneous  con- 
tractions.) When  we  observe  the  effect  of  electric 
shocks  upon  the  ventricle  we  note  an  apparent  dif- 
ference between  its  response  and  that  of  skeletal  muscle. 
The  contractions  of  a  skeletal  muscle  are  proportional 
in  height  to  the  strength  of  the  shocks  employed  to  pro- 
duce them,  within  a  certain  range  of  intensity.  The 
heart  muscle,  on  the  contrary,  is  said  to  obey  the  All 
or  None  Principle.  If  any  response  follows  a  shock  it 
is  a  typic,  full-sized  contraction.  We  cannot  cause  the 
ventricle  to  execute  small  beats  by  reducing  the  strength 
of  our  stimulation  to  a  minimum. 

A  satisfactory  explanation  of  the  difference  between 
skeletal  muscle  with  its  graded  responses  and  cardiac 
muscle  with  its  uniform  contractions  can  probably  be 
given.  The  cells  of  the  cardiac  tissue  are  so  linked  that 
the  activity  of  one  excites  its  neighbors.  A  disturbance 
once  initiated  will  sweep  through  the  whole  fabric.  The 
fibers  of  skeletal  muscle  are,  so  to  speak,  insulated  and 
the  contraction  of  certain  ones  does  not  tend  to  bring 
others  into  action.  It  has  therefore  been  urged  that  the 
small  contractions  often  made  by  skeletal  muscles  are 
due  to  the  contraction  of  a  small  proportion  of  the 
fibers,  the  great  majority  remaining  idle. 

Cardiac  muscle  does  not,  as  a  rule,  show  summation 
or  tetanus  when  stimulated  in  a  manner  which  would 
develop  these  phenomena,  in  skeletal  muscle.  Rapidly 
repeated  shocks  do  not  throw  the  heart  into  intense  and 
continuous  contractions  but  cause  it  to  flutter.  The 
fact  seems  to  be  that  whenever  the  cardiac  muscle  goes 
into  contraction  it  expends  all  its  resources  for  the 
moment  and  is  bound  to  relax  somewhat  while  it  is  re- 


THE    HEART  259 

gaining  power  to  work  again.  Hence,  after  an  effective 
stimulus  has  been  administered  nothing  is  gained  by 
repetition  until  diastole  has  set  in.  The  heart  is  said 
to  be  refractory  toward  stimulation  while  it  is  in  its 
systolic  phase. 

The  Pulse. — What  we  call  the  pulse  is  a  momentary 
swelling  which  we  notice  in  an  artery.  It  is  associated 
with  the  intermittent  movement  of  the  blood-stream 
discussed  in  the  previous  chapter,  but  it  stands  in  such 
a  relation  to  the  heart  action  that  consideration  has  been 
postponed  until  now.  The  swelling  which  is  felt  occurs 
after  each  systole  of  the  left  ventricle.  It  is  the  sign  of 
the  transient  increase  of  arterial  content  due  to  the 
introduction  of  blood  from  the  heart.  The  lateral  en- 
largement occurs  first  in  the  aorta,  but  is  shifted  with 
great  speed  to  all  the  branches  of  the  arterial  tree.  In 
thus  shifting  along  the  walls  of  the  elastic  vessels  the 
pulse  has  a  wave-like  character. 

On  the  surface  of  a  river  the  ripples  may  run  upstream, 
across,  or  downstream  and  at  rates  entirely  unrelated 
to  that  of  the  current.  So,  in  the  case  of  the  pulse,  we 
have  to  recognize  that  it  travels  at  a  speed  which  is  of 
an  utterly  different  order  from  that  of  the  blood-stream. 
In  fact  the  pulse  is  propagated  many  times  faster  than 
the  blood.  A  corpuscle  may  take  four  or  five  seconds  to 
run  from  the  semilunar  valves  to  the  wrist,  but  the  wave 
of  swelling  will  pass  over  the  same  distance  in  a  fraction 
of  one  second. 

The  physician  who  feels  the  pulse  with  his  practised 
fingers  can  learn  from  it  not  only  the  frequency  and 
regularity  of  the  heart-beat,  but  whether  the  output  at 
each  systole  is  large  or  small  and  whether  the  prevail- 
ing average  pressure  is  high  or  low.  Still  other  signs 
of  the  condition  of  the  circulation  may  be  apparent  to 
him.  If,  for  example,  the  aortic  semilunar  valves  do 
not  close  effectively,  the  pulse  is  said  to  have  a  "  collaps- 
ing" character,  the  swelling  disappears  and  the  tension 
is  eased  with  abnormal  abruptness. 


CHAPTER  XIX 
THE  REGULATION  OF  THE  CIRCULATION 

We  have  treated  the  subject  of  the  circulation  thus 
far  from  a  mechanical  point  of  view.  We  have  dealt 
with  average  conditions  as  they  may  be  visualized  for 
the  human  being,  and  we  have  said  little  or  nothing  of 
the  adaptive  changes  which  take  place  from  time  to 
time.  But  the  reader  has  learned  that  adaptive  changes 
are  to  be  looked  for  in  any  living  system  and  perhaps 
they  are  nowhere  more  striking  than  in  the  heart  and 
the  blood-vessels.  The  facts  have  been  hinted  at  in 
Chapter  VIII;  a  fuller  exposition  is  now  called  for. 

The  regulation  of  the  blood-flow  may  be  considered 
under  two  heads.  We  must  give  an  account  of  the 
influence  exerted  through  the  nerve  centers  upon  the 
heart,  and  we  must  later  consider  the  vasomotor  mechan- 
ism. Under  the  first  head  we  deal  with  the  total 
quantity  of  blood  pumped  and  under  the  second  we 
attend  to  its  varying  distribution. 

We  have  properly  emphasized  the  automatic  character 
of  the  heart.  But  we  must  not  ignore  the  influence  of 
the  nervous  system  upon  its  rate  and  force.  The  heart 
is  like  a  horse  in  harness:  it  is  seen  to  be  working  at 
a  certain  rate  and  there  is  no  doubt  of  its  independent 
power  to  act,  but  we  cannot  say  that  it  is  doing  quite 
what  it  would  do  if  left  free  from  all  guidance.  To 
destroy  the  brain  is  not  to  stop  the  heart,  but  a  change 
in  its  rate  under  the  circumstances  is  to  be  expected. 
In  the  same  way  a  horse  may  go  on  his  way  when  his 
driver  has  fallen  from  his  seat,  but  a  change  in  his  pace 
is  highly  probable. 

The  statement  has  already  been  made  that  the 
severing  of  connections  between  the  nervous  system 

260 


THE    HEART  261 

and  the  heart  is  most  likely  to  result  in  a  quickening 
of  the  beat.  This  indicates  that  the  prevailing  influence 
of  the  centers  is  inhibitory.  The  paths  of  the  nerve- 
impulses  by  which  the  heart  is  thus  restrained  from 
"running  away"  are  in  the  two  vagus  nerves.  These 
nerves  have  been  described  as  being  the  tenth  pair  in 
the  cranial  series.  They  have  many  other  functions 
besides  that  of  cardiac  inhibition,  though  this  is  the 
one  the  student  is  most  apt  to  associate  with  them. 

The  discovery,  in  1845,  that  the  heart  is  inhibited  when 
the  vagus  nerve  is  stimulated  was  one  of  the  most  sig- 
nificant in  the  history  of  physiology.  It  showed  that  the 
activity  of  living  tissues  can  be  restrained  as  well  as 
excited  through  nerves  leading  to  them.  This  is  a 
conception  which  it  was  most  important  for  the  scientist 
to  grasp  and  it  remains  to  this  day  unfamiliar  to  the 
layman.  Cardiac  inhibition  is  quite  different  from  the 
inhibition  of  the  tone  of  skeletal  muscles  which  was 
described  in  connection  with  reciprocal  innervation 
(Chapter  VI).  With  the  heart  it  is  the  contractile  sub- 
stance which  is  prevented  from  full  activity.  In  the 
other  case  the  restraint  is  applied  to  certain  nerve  cells. 

The  facts  of  cardiac  inhibition  were  first  observed  in 
the  frog.  The  heart  of  this  animal  stops  beating  when 
the  vagus  is  stimulated  and  can  be  kept  at  rest  for  an 
indefinite  time.  Anyone  noting  the  behavior  of  the 
heart  for  the  first  time  would  be  likely  to  infer  that  the 
muscle  had  been  thrown  into  a  sustained,  cramp-like 
contraction  resembling  the  tetanus  of  the  skeletal  type. 
That  is  to  say,  he  would  think  that  the  beating  of  the 
heart  ceased  because  no  diastole  occurred.  However, 
close  inspection  makes  it  plain  that  the  inhibited  heart 
is  in  diastole  and  not  systole;  it  is  not  prevented  from, 
relaxing  but  from  contracting. 

Complete  inhibition  of  the  heart  involves  complete 
arrest  of  the  circulation.  This  would  result  in  death 
within  a  few  minutes  in  any  of  the  warm-blooded  animals. 
But  it  is  scarcely  possible  to  kill  a  frog  or  a  turtle  by  this 


262  HUMAN    PHYSIOLOGY 

means.  The  oxygen  requirement  of  the  tissues  is  so 
very  moderate  that  fatal  asphyxia  does  not  take  place 
either  in  the  heart  itself  or  elsewhere.  The  heart  of  a 
turtle  has  been  held  motionless  by  vagus  stimulation 
for  four  hours  and  has  resumed  beating  when  the  arti- 
ficial restraint  has  been  removed.  When  we  make  ex- 
periments upon  cats  and  dogs  we  find  that  it  is  not 
possible  to  kill  these  animals  either  by  inhibition  of  the 
heart.  Life  is  preserved  through  the  trial  because  the 
warm-blooded  heart  soon  resumes  beating  in  spite 
of  the  application  of  stimuli  to  the  vagus.  The  heart, 
in  such  a  case,  is  said  to  " escape"  from  inhibition. 

The  extent  to  which  the  heart  can  be  inhibited  varies 
greatly  in  different  species.  In  the  cat  it  can  scarcely 
be  stopped  at  all,  but  it  can  be  slowed  and  thus  its  output 
can  be  diminished.  Perhaps  the  student  is  somewhat 
misled  by  the  striking  experiment  of  stopping  the  heart. 
He  is  to  consider  that  the  laboratory  trial  does  not  illus- 
trate a  normal  function  but  an  exaggeration  of  one. 
It  could  never  be  advantageous  to  arrest  the  heart  of 
any  animal.  So  we  must  think  that  the  true  service 
of  the  mechanism  reached  by  the  vagus  fibers  is  to 
secure  economy  and  to  provide  a  reserve  for  emergencies. 

The  point  has  been  made  that  the  average  condition 
of  the  living  heart  is  one  in  which  some  degree  of  vagus 
inhibition  is  operative.  We  say  that  the  vagus  influence 
is  a  " tonic"  or  sustained  one.  If  this  is  true,  it  follows 
that  a  heart  which  is  speeding  up  may  be  displaying  the 
results  of  a  withdrawal  of  the  usual  restraint.  Its  quick- 
ening may  be  like  the  acceleration  of  the  train  on  the 
down  grade  when  the  brakes  are  taken  off.  The  nervous 
system  is  n'ot  hurrying  the  heart  but  merely  giving  it 
liberty  to  make  its  own  pace. 

Nevertheless,  the  heart  is  provided  with  nervous  con- 
nections through  which  it  can  be  definitely  excited  to 
work  more  rapidly  and  more  powerfully.  These  con- 
nections are  the  accelerator  nerves  of  the  heart.  They 
reach  it  otherwise  than  by  the  vagus,  and  the  general 


THE    REGULATION    OF    THE    CIRCULATION       263 

statement  may  be  made  that  they  come  from  certain 
ganglia  upon  the  dorsal  wall  of  the  thorax.     The  cells 


FIG.  57. — A  diagrammatic  representation  of  the  heart's  nervous 
connections,  (x-x-x)  is  the  vagus,  descending  from  the  medulla  to 
the  abdomen  and  giving  inhibitory  fibers  to  the  heart;  (g-g-g-g)  are 
ganglia  of  the  so-called  "sympathetic"  chain,  supplying  accelerators  to 
the  heart.  Only  the  nerves  in  the  left  half  of  the  body  are  illustrated. 

in  these  ganglia  are  under  the  influence  of  nerve  fibers 
from  the  neighboring  spinal  cord.  A  skilled  operator 
can  pick  up  slender  strands  between  the  ganglia  and 


264  HUMAN    PHYSIOLOGY 

the  heart  and  show,  by  means  of  stimulation,  that  it  is 
possible  through  them  to  increase  the  force  and  fre- 
quency of  its  beating.  The  effect  is  not  very  striking. 

Cardiac  Regulation.  Summary. — The  heart  has  an 
automatic  tendency,  particularly  localized  in  the 
auricles,  which  would  determine  its  rate  in  the  absence 
of  external  influences.  But  the  ordinary  rate  is  not  that 
which  the  auricles  would  spontaneously  determine.  It 
is  considerably  slower  than  this  and  results  from  a  tonic 
inhibitory  action  exerted  by  the  medulla  through  certain 
fibers  in  the  vagus  nerves.  This  tonic  action  may  be 
abated,  causing  the  heart  to  quicken  for  a  time,  and  may 
then  be  resumed.  Finally,  there  is  provision  for  a 
moderate  acceleration  of  the  heart  by  impulses  follow- 
ing definite  nerve  paths.  It  will  be  seen  that  it  is  hard 
to  be  sure  whether  a  heart  which  is  quickening  its  beat 
is  exhibiting  the  withdrawal  of  the  vagus  effect  or  the 
active  intervention  of  the  accelerators. 

Vasomotor  Control. — It  has  been  said  that  the  large 
arteries  are  essentially  elastic  tubes  while  the  smallest 
branches  of  the  arterial  tree  .are  well  provided  with 
contractile  elements.  The  swelling  of  large  arteries 
occurs  when  the  pressure  of  the  blood  within  has  been 
raised  and  need  not  be  referred  to  any  living  character- 
istic of  these  vessels.  The  finer  subdivisions  (arterioles) 
may  contract  against  a  rising  pressure  or  dilate  when 
the  pressure  is  falling.  These  possibilities  exist  only 
because  the  vessels  in  question  are  largely  composed 
of  living,  contractile  tissue.  We  may  compare  the  large 
trunks  of  the  arterial  system  with  rubber  tubes  and  not 
be  seriously  misled,  but  we  cannot  extend  the  com- 
parison to  the  arterioles. 

The  smooth  muscle  of  the  arterioles  is  reached  by 
numerous  nerve  fibers.  Under  their  influence  its  tons 
may  be  increased  or  diminished.  Such  are  called  vaso- 
motor  fibers  and  we  often  speak  of  vasornotor  nerves. 
Strictly  speaking,  what  we  mean  by  a  vasomotor  nerve 
is  usually  a  nerve  in  which  vasomotor  fibers  abound; 


THE    REGULATION    OF   THE    CIRCULATION       265 

we  cannot  ordinarily  assume  that  no  other  kinds  of 
fibers  are  present.  It  will  make  for  clearness  if  a  classic 
experiment  is  now  described. 

More  than  sixty  years  ago  the  eminent  French 
physiologist,  Bernard,  severed  a  certain  nerve  in  the  neck 
of  a  rabbit  and  witnessed  the  curious  effect  manifested  in 
the  ear  of  the  animal.  If  the  ear  of  a  white  rabbit  is  held 
up  to  the  light  the  red  network  of  the  blood-vessels  will 
be  conspicuous.  Bernard  found  that  a  general  enlarge- 
ment of  the  vessels  in  the  ear  followed  the  cutting  of 
the  nerve.  It  is  now  known  that  the  nerve  operated 
on,  the  cervical  sympathetic,  contains  fibers  which  in- 
fluence the  muscle  cells  in  the  walls  of  these  vessels. 

What  interpretation  is  to  be  placed  upon  Bernard's 
experiment?  We  believe  that. the  enlargement  of  the 
vessels  indicates  that  they  were  previously  held  in  a 
state  of  partial  contraction  maintained  by  the  constant 
arrival  of  impulses  from  the  central  nervous  system. 
With  the  cutting  of  the  vasomotor  fibers  the  vessels 
were  paralyzed,  their  tone  was  lost.  Very  soon  after 
the  original  observation  the  complementary  discovery 
was  made  that  electric  stimulation  of  the  cut  nerve  would 
restore  the  tone  of  these  vessels  and,  in  fact,  reinforce 
it  beyond  the  normal  state,  causing  the  ear  to  blanch. 
The  fibers  whose  existence  is  thus  demonstrated  are 
logically  called  vasoconstrictors.  The  term  implies  that 
they  convey  impulses  which  make  the  vessels  contract. 
Such  contraction  naturally  reduces  the  local  blood- 
supply. 

It  has  become  evident  since  the  time  of  Bernard  that 
vasoconstrictor  fibers  have  a  most  extensive  distribution 
in  the  animal  body.  If  a  nerve  is  chosen  at  random 
and  stimulated,  it  is  usual  to  see  signs  of  a  lessened 
blood-supply  in  the  region  influenced  by  the  nerve. 
This  is  most  strikingly  true  in  the  case  of  a  pair  of 
nerves,  the  splanchnics,  which  take  their  rise  from 
certain  ganglia  at  the  back  of  the  thorax  and  pass  down 
through  the  diaphragm  to  be  distributed  in  the  ab- 


266  "HUMAN    PHYSIOLOGY 

domen.  Stimulation  of  the  splanchnic  nerves  greatly 
reduces  the  quantity  of  blood  passing  through  the 
vessels  of  the  digestive  tract.  Cutting  the  splanchnics 
permits  these  vessels  to  widen  according  to  the  principle 
of  Bernard's  experiment. 

If  we  abolish  arterial  tone  in  an  important  field  like 
that  served  by  the  splanchnic  nerves  we  shall  greatly 
lower  the  pressure  in  .the  aorta  and  its  branches.  There 
are  two  reasons  for  this:  first,  the  widened  vessels  op- 
pose less  resistance  to  the  onward  movement  of  the 
blood  and,  second,  the  capacity  of  the  abdominal  vessels 
is  so  much  increased  that  they  retain  an  abnormal 
quantity  of  blood,  leaving  so  much  less  to  circulate 
elsewhere.  This  has  been  described  as  "  bleeding  into 
one's  own  veins"  and  is  a  common  cause  of  faintness, 
for  example,  in  severe  indigestion. 

Vasodilator  Fibers. — The  experiment  of  stimulating 
a  nerve  to  observe  the  effect  upon  the  blood  flow  in  its 
field  does  not  always  give  the  constrictor  reaction. 
We  have  evidence  of  the  existence  of  an  opposing  order 
of  vasomotor  fibers  through  which  the  dilation  of  small 
blood-vessels  can  be  promoted.  Such  are  the  vaso- 
dilator fibers.  If,  for  example,  we  stimulate  the  small 
nerve  known  as  the  chorda  tympani,  an  offshoot  of 
the  seventh  cranial,  we  may  see  that  the  blood-flow 
through  the  submaxillary  gland  instead  of  being  reduced 
is  much  increased.  The  result  is  explained  by  the 
assumption  that  there  are  fibers  in  this  nerve  which 
are  inhibitory  to  the  contractile  elements  in  the  asso- 
ciated vessels. 

Students  often  find  it  hard  to  visualize  the  action  of 
vasodilator  fibers.  It  may  be  admitted  that  it  is  not 
a  simple  matter.  How  can  stimulation  of  nerve  fibers 
increase  the  diameter  of  arteries?  We  shall  probably 
do  well  to  emphasize  the  fact  of  the  internal  pressure  in 
this  connection.  An  artery  will  dilate  if  the  usual  re- 
sistance to  this  pressure  is  diminished.  The  chemical 
changes  which  occur  in  the  muscle  cells  under  the  in- 


THE    REGULATION    OF   THE    CIRCULATION       267 

fluence  of  the  vasodilator  fibers  are  of  a  paralyzing  sort, 
and  the  vessel  is  left  free  to  become  stretched  by  the 
stream  that  is  being  driven  through  it. 

It  will  now  be  appreciated  that  the  blood-vessels,  like 
the  heart,  may  be  played  upon  by  nerve-impulses  having 
opposite  effects.  The  vagus  fibers  inhibit  the  heart; 
the  dilator  fibers  inhibit  the  blood-vessels.  The  heart 
has  accelerator  fibers  which  reinforce  its  contractile 
activity  and  the  tone  of  the  small  vessels  can  be  rein- 
forced through  the  constrictor  fibers.  It  should  be 
added  that  in  the  case  of  the  heart  the  inhibitory  in- 
fluence is  more  in  evidence  than  the  opposing  one,  while 
the  constrictor  effect  is  more  prominent  than  the  dilator 
among  the  small  vessels.  Vasodilators  do  not  seem  to 
have  a  universal  distribution.  Their  existence  is  best 
established  for  the  glands,  the  muscles,  the  skin  of  the 
face,  and  the  genitals. 

Vasomotor  Adjustments. — We  are  now  in  a  position 
to  consider  some  of  the  adaptive  changes  made  by  the 
nervous  mechanisms  presiding  over  the  circulation. 
The  great  purpose  to  be  subserved  is  to  provide  an  ample 
supply  of  blood  to  the  various  capillary  areas  and  to 
meet  the  varying  demands  of  different  regions.  On 
the  negative  side  we  may  recognize  that  economy  is 
secured  by  restricting  the  flow  through  parts  where 
there  is  no  pressing  need.  The  general  principle  is  to 
increase  the  supply  to  any  seat  of  activity,  motor  or 
secretory,  and  to  limit  the  amount  of  blood  sent  through 
tissues  which  are  temporarily  at  rest. 

A  local  demand,  such  as  may  be  made  by  the  salivary 
glands  during  mastication,  may  be  satisfied  by  a  lower- 
ing of  the  tone  in  the  vessels  of  these  organs  and  may 
not  entail  an  appeal  to  the  heart.  A  widespread  need, 
like  that  of  muscular  exercise,  requires  not  only  the 
dilation  of  many  vessels,  but  a  general  speeding  up  of 
the  circulation  which  can  only  be  maintained  by  in- 
creased heart  action.  (When  vigorous  exercise  is  taken 
the  dilation  must  extend  to  the  skin  as  well  as  the  muscles. 


268  HUMAN    PHYSIOLOGY 

The  object  is  to  promote  the  dissipation  of  the  extra 
heat  which  is  being  produced.) 

In  a  later  chapter  we  shall  discuss  the  regulation  of 
body  temperature.  At  this  time  we  ought  to  point  out 
briefly  that  the  vasomotor  system  plays  an  important 
part  in  this  necessary  work.  When  the  surface  of  the 
body  is  warmed  a  familiar  vasomotor  reflex  leads  to  the 
enlargement  of  the  cutaneous  vessels  and  the  quantity 
of  blood  near  the  cooling  atmosphere  is  augmented. 
Under  the  influence  of  cold  we  find  that  the  circulation 
in  the  skin  is  hindered.  Moderate  cooling  of  the  sur- 
face leads  to  paling.  But  we  must  use  a  little  space  to 
comment  on  the  effect  of  more  decided  chilling.  Every- 
one knows  that  this  produces  a  reddening,  and  it  might 
be  supposed  that  the  underlying  condition  would  be 
the  same  as  that  of  the  skin  when  flushed  with  heat. 
The  actual  state  of  affairs  seems  to  be  quite  different. 

Suppose  that  one  hand  is  held  for  a  while  in  ice  water 
while  the  other  is  in  water  as  hot  as  can  be  borne.  Both 
will  probably  be  red,  but  we  can  discover  signs  that  the 
existing  conditions  are  not  identical.  It  will  be  seen 
that  the  veins  in  the  heated  hand  are  swollen  while  in 
the  chilled  hand  they  are  small.  The  redness  induced 
by  cold  does  not  seem  to  be  due  to  a  copious  flow  of 
blood  through  the  skin  but  to  a  gathering  of  relatively 
stagnant  blood  in  the  capillaries.  The  venous  outlets 
appear  to  be  contracted.  This  is  in  agreement  with 
the  fact  that  the  skin  which  has  been  reddened  by  cold 
may  easily  become  bluish.  The  bluish  cast  is  always 
the  sign  of  an  impeded  rather  than  an  accelerated 
circulation. 

This  illustration  makes  it  convenient  to  speak  briefly 
of  congestions.  When  there  is  an  excess  of  blood  in 
a  part  we  say  that  there  is  a  local  congestion.  But 
congestions  are  of  two  types  and  fairly  exemplified  by 
the  states  produced  by  heating  and  cooling  the  skin. 
First,  we  have  the  active  type  in  which  the  arteries  are 
dilated,  admitting  more  blood  than  usual  to  the  capil- 


THE    REGULATION    OF    THE    CIRCULATION       269 

laries;  while  there  is  no  hindrance  to  its  free  escape 
through  the  veins.  Second,  there  is  the  passive  type  in 
which  the  outstanding  feature  is  the  narrowing  or  the 
obstruction  of  the  veins.  The  result  is  an  increased 
quantity  of  blood  in  the  capillaries,  but  it  is  more  or 
less  completely  arrested  or  dammed  up.  This  is  the 
case  when  cold  has  had  its  usual  effect  on  the  skin. 

We  have  emphasized  the  part  played  by  the  small 
arteries  in  vasomotor  regulation.  The  small  veins  have 
a  less  marked  but  still  noteworthy  muscular  develop- 
ment and  there  is  no  doubt  that  they  have  a  share  in  the 
adaptations.  But  this  is  seldom  given  much  prominence 
by  writers  on  the  subject  and  there  is  much  uncertainty 
in  regard  to  its  extent.  The  passive  congestions  of  path- 
ology are  not  due  to  muscular  constriction  of  the  veins  so. 
much  as  to  internal  accumulations  of  cells,  chiefly  white 
corpuscles.  Complete  closure  of  the  veins  in  any  region 
will  throw  the  full  arterial  pressure  into  the  capillaries, 
distending  them  severely  and  probably  increasing  the 
formation  of  lymph  as  the  fluid  filters  out  into  the  tissue 
spaces. 

Colds. — What  has  just  been  said  of  congestion  is 
related  to  our  susceptibility  to  common  colds.  We 
speak  of  taking  or  catching  cold,  and  it  has  become 
probable  that  there  are  two  senses  in  which  we  may 
do  just  this  thing.  A  cold  is  almost  certainly  an  in- 
fectious disease;  as  such  one  may  take  it  from  a  previous 
victim.  But  we  also  speak  of  catching  cold  when  we 
fix  the  blame  on  circumstances  under  which  we  have 
been  chilled  by  drafts  or  by  getting  wet.  We  shall 
find  that  the  recognition  of  the  infectious  nature  of 
colds  does  not  lead  to  the  denial  of  the  other  mode  of 
acquiring  them. 

When  the  surface  of  the  body  is  cooled  it  is  entirely 
normal  for  the  mucous  membranes  to  become  engorged 
with  blood.  An  active  congestion,  more  or  less  marked, 
is  to  be  expected.  This  may  result  in  increased  secre- 
tion on  the  part  of  the  nasal  lining.  But  a  hardy  and 


270  HUMAN    PHYSIOLOGY 

well-conditioned  person  has  a  vasomotor  system  which 
can  be  relied  on  to  abate  this  congestion  in  a  prompt 
and  decisive  manner  when  the  external  conditions  are 
changed.  Another,  less  highly  endowed  in  this  re- 
spect, may  not  terminate  the  congestion  in  the  nose 
with  such  success.  It  may  linger 'until  the  tissue  is 
definitely  injured  or  inflamed,  when  it  will  tend  to 
become  passive  rather  than  active  in  character. 

A  passive  congestion  is  always  deleterious  to  the 
local  cell-life.  The  mucous  membrane  beneath  which 
the  circulation  is  thus  interrupted  becomes  fallow 
ground  for  the  growth  of  microorganisms.  There  are 
ordinarily  enough  of  these  present  to  furnish  seed  for 
an  infection.  So  a  cold  may  develop  which  does  not 
appear  to  have  been  " taken"  from  another  person. 
The  symptoms  may  be  due  to  the  activity  of  bacteria, 
but  the  occasion  for  this  ebullition  of  germ  life  has 
been  furnished  by  the  lowered  resistance  of  the  con- 
gested tissue. 

When  we  attribute  freedom  from  colds  to  "  vital  re- 
sistance" our  reference  may  be  to  either  of  two  condi- 
tions. We  may  have  in  mind  a  chemical  peculiarity 
of  the  blood  and  other  fluids  which  confers  on  them 
the  power  to  destroy  bacteria.  We  are  quite  as  likely 
to  be  picturing  an  organization  which  is  resistant  be- 
cause of  its  vasomotor  efficiency.  The  hardiness  gained 
by  athletic  training  and  cold  bathing  is  probably  not 
a  chemical  superiority  but  a  mechanical  one.  The 
merit  lies  less  in  the  composition  of  the  blood  than  in 
the  positive  and  consistent  government  of  the  circula- 
tion. Colds  are  not  contracted  because  incipient  con- 
gestions are  overcome  by  the  vigorous  constriction  of 
the  arteries  immediately  after  the  period  of  exposure. 

The  Cranial  Blood-supply. — It  might  seem  most 
difficult  to  learn  anything  about  the  changes  taking 
place  in  the  blood-vessels  of  the  brain.  The  surface 
of  the  cerebrum  has  sometimes  been  observed  after 
extensive  fractures  of  the  skull,  and  it  hu,s  been  reported 


THE    REGULATION    OF    THE    CIRCULATION       271 

that  the  vessels  there  become  less  conspicuous  in  sleep 
than  they  are  in  waking.     Quite  aside  from  these  rare 

indirect  evidence  in  regard 

local  prv 


'••>r  the 

• 


11    • 

table 

in  the  extre  part 

•  y.     The    instrumen1  i    the 

• 

The    pleth 3-  o 


272  HUMAN    PHYSIOLOGY 

. 

. 

- 

. 
- 

hand 
More 

the  amount  o 

• 

y    condition.      • 

• 
letic. 


' 

. 

^ 

It'       A 

tent  ion    i- 

part 

serviceable  to  the  biain.     The  contraction  of  in  _____ 

arteries  will  elevate  the  pressure  in  the  large  trunks  and 

there  will  be  a  swifter  flow  of  blood  through  any  vessels 


THE    REGULATION    OF    THE    CIRCULATION       273 

which  do  not  share  in  the  constriction.  It  is  just  as 
though  one  of  two  faucets  over  a  sink  should  be  turned 
off;  there  would  be  a  reinforcement  of  the  discharge 
from  the  one  that  was  left  open. 

The  act  of  falling  asleep  is  psychologically  the  reverse 
of  fixing  the  attention;  it  is  dissipation  instead  of  con- 
centration. It  can  be  shown  by  means  of  the  plethys- 
mograph  that  it  is  also  the  reverse  in  its  physical  signs. 
Individuals  have  schooled  themselves  to  sleep  with  the 
arm  in  the  plethysmograph  and  the  records  obtained 
show  that  about  the  time  consciousness  is  lost,  or  trans- 
formed to  that  of  the  world  of  dreams,  there  is  a  slack- 
ening of  tone  in  the  vessels,  an  increase  of  volume  in 
the  limb.  If  what  happens  in  the  arm  is  typical  of 
what  takes  place  elsewhere  there  must  be  a  lowering 
of  pressure  in  the  main  arteries  at  such  a  time.  Then, 
if  the  vessels  of  the  brain  do  not  greatly  change  their 
caliber  the  flow  through  them  will  be  much  reduced. 
When  sleep  was  under  discussion  in  Chapter  XII  it 
was  stated  that  diminished  blood-flow  might  account 
for  its  onset. 

Of  course  the  act  of  waking  is  a  resumption  of  atten- 
tion. It  is  known  to  be  attended  by  a  constriction  of 
the  vessels  in  the  part  observed.  Thus  all  that  we 
know  of  the  adjustments  in  favor  of  the  brain  is  easily 
coordinated  and  understood.  The  reciprocal  element 
in  vasomotor  changes  is  apparent  in  other  cases  besides 
this.  We  assume  that  a  dilation  of  the  surface  vessels 
will  relieve  internal  congestion.  This  is  the  prfncfple 
which  justifies  hot  applications.  An  obvious  feature  of 
the  reaciSfch  to  alcohol  is  the  flushing  of  the  skin,  and 
here  too  we  suppose  that  there  is  a  corresponding  with- 
drawal of  blood  from  the  deeper  parts. 

The  Afferent  Side. — So  far  we  have  spoken  of  the 
cardiac  and  vasomotor  nerves  as  pathways  leading 
from  the  central  gray  matter  to  the  organs  of  the  cir- 
culation. Little  has  been  said  to  bring  out  the  fact  that 
they  are  used  in  reflex  fashion.  But  the  general  truth 

18 


274  HUMAN    PHYSIOLOGY 

is  always  to  be  borne  in  mind  that  efferent  nerves  carry 
out  from  the  brain  and  cord  impulses  which  owe  their 
origin  to  the  previous  arrival  of  other  impulses  over 
sensory  paths.  There  is  therefore  an  afferent  as  well 
as  an  efferent  side  to  this  mechanism. 

A  series  of  light  blows  rapidly  delivered  upon  the 
abdomen  of  a  frog  may  slow  or  even  stop  the  heart. 
This  is  a  clear  case  of  reflex  inhibition.  Impulses  run 
to  the  medulla  and  cause  the  return  of  others  which 
find  their  way  to  the  heart  along  the  vagus  fibers.  In 
like  manner,  afferent  impulses  may  reflexly  affect  the 
tone  of  the  blood-vessels,  either  slackening  or  tightening 
the  arteries.  A  reflex  change  in  which  arterial  tone  is 
lowered  is  called  a  depressor  reaction;  the  contrasted 
reflex,  with  heightening  of  tone,  is  a  pressor  response. 
The  adjustment  made  to  support  the  fixation  of  the 
attention  and  demonstrated  with  the  plethysmograph 
is,  accordingly,  a  pressor  reflex. 

The  average  aortic  pressure  is  kept  sufficient  to  insure 
an  adequate  blood-supply  wherever  a  channel  may  be 
opened  by  vasodilation.  It  is  important  that  this 
pressure  shall  not  greatly  fall,  and  it  is  in  the  interests 
of  economy  and  safety  that  it  shall  not  greatly  rise. 
An  elaborate  reflex  mechanism  exists  to  keep  it  within 
reasonable  limits.  If  the  pressure  tends  to  mount  up, 
of  course  one  of  the  direct  results  will  be  a  stretching 
of  the  aorta.  But  there  are  receptors  in  the  wall  of 
this  great  vessel  which  are  stimulated  when  the  high 
tension  is  established.  Impulses  are  set  up  in  nerve 
fibers  which  lead  to  the  medulla.  Reflex  responses  follow 
which  abate  the  rising  pressure. 

The  reflexes  just  referred  to  are  of  two  kinds.  Some 
degree  of  inhibition  is  imposed  upon  the  heart  and  there 
is  simultaneously  a  lowering  of  tone  in  the  vessels  of 
the  digestive  tract.  Thus  the  high  arterial  tension  is 
relieved  both  by  easing  the  heart,  and  so  reducing  its 
output,  and  by  providing  for  the  freer  escape  of  the 


THE    REGULATION    OF    THE    CIRCULATION       275 

blood  from  the  distended  arteries  to  the  veins.  This  is 
one  of  the  most  impressive  examples  we  have  of  a 
complex  nervous  mechanism  devoted  to  the  fulfillment 
of  an  important  function  and  quite  shut  away,  under 
most  circumstances,  from  our  own  intermeddling. 


CHAPTERjgi, 
BREATHING 

Respiration  has  been  defined  earlier  in  this  book  as 
the  oxidation  process  which  accompanies  life  and  serves 
to  release  the  potential  energy  of  the  physiologic  fuels. 
In  this  intimate  sense  respiration  is  best  studied  as  a 
part  of  metabolism,  the  chemistry  of  the  living  matter. 
But  while  the  facts  of  the  circulation  are  fresh  in  mind 


FIG.  59. — A  microscopic  unit  (air-cell)  from  the  lung  tissue.  The 
capillaries  are  wrapped  about  a  sac  the  form  of  which  may  be  likened 
to  that  of  a  blackberry.  In  the  drawing  this  sac  is  represented  as 
partly  laid  open  and  the  minute  bronchial  tube  leading  to  it  is  slit. 
The  rent  shows  the  concavities  of  the  interior  surface. 

we  shall  do  well  to  enter  upon  a  treatment  of  breathing 
and  the  carriage  of  gases  in  the  blood.  Breathing  is 
not  respiration,  in  the  best  use  of  that  word,  but  it  is 
preliminary  to  it  and  also  subsequent  to  it;  it  introduces 
the  oxygen  into  the  blood  and  provides  for  the  escape 
of  the  carbon  dioxid  into  the  air. 

The  Respiratory  Apparatus. — It  has  been  said  already 
that  the  lungs  are  organs  in  which  the  practical  con- 
tact of  blood  and  air  can  be  maintained.  The  pul- 

276 


BREATHING  277 

monary  arteries  conduct  the  blood  from  the  right  side 
of  the  heart  into  capillaries  which  are  wrapped  about 
minute  air-sacs.  Two  cell-layers  are  here  interposed 
between  the  blood  and  the  air.  The  gases  have  to  pass 
through  the  capillary  wall  and  also  the  wall  of  the  air- 
sac.  But  the  wall  of  the  capillary  is  of  the  utmost 
delicacy  and  the  epithelium  of  the  air-sac  is  of  the  same 
nature.  If  the  blood  actually  fell  in  drops  through  an 
air-chamber  it  would  probably  be  no  more  thoroughly 
aerated  than  it  is  in  fact. 

Each  lung  is  to  be  regarded  as  consisting  of  a  system 
of  blood-vessels  intertwined  with  vessels  containing  air. 
The  resulting  tissue  is  peculiarly  light  and  yielding. 
It  is  for  this  reason  that  the  lungs  of  the  lower  animals 
are  colloquially  called  "the  lights."  If  they  are  thrown 
upon  water  they  float  with  a  great  part  of  their  mass 
above  the  surface.  The  two  lungs  fill  the  greater 
portion  of  the  chest  cavity.  The  left  lung  has  a  some- 
what smaller  volume  than  the  right,  chiefly  because  the 
heart  subtracts  space  from  that  half  of  the  thorax. 
On  the  other  hand,  the  left  lung  is  rather  longer  from  top 
to  bottom  than  its  fellow.  Roughly  speaking,  the  two 
taken  together,  with  the  heart  and  the  connecting  vessels, 
form  a  cone  with  its  base  upon  the  diaphragm. 

The  air  which  we  breathe  reaches  and  leaves  the  lungs 
by  way  of  the  trachea.  This  is  a  tube  extending  down- 
ward from  the  larynx  which,  it  will  be  recalled,  is  at 
the  root  of  the  tongue  and  ventral  to  the  upper  part 
of  the  esophagus.  The  trachea  continues  in  front  of 
the  esophagus  to  a  point  within  the  chest  where  it 
forks  into  branches,  the  bronchi,  leading  into  the  two 
lungs.  The  trachea  and  the  bronchi  have  rather  rigid 
walls  and  are  kept  open  at  all  times.  This  firmness  is 
owing  to  bows  of  cartilage  imbedded  in  their  tissues. 
The  contrast  between  these  carriers  of  air  and  the 
esophagus  is  marked,  for  the  latter  is  nearly  or  quite 
collapsed  unless  something  is  passing  through  it. 

Air  may  reach  the  larynx  through  the  mouth  or  the 


278 


HUMAN    PHYSIOLOGY 


nose.  The  nasal  passages  occupy  a  large  space  in  the 
skull  and  are  extensively  subdivided.  Scroll-like  proc- 
esses from  their  walls  jut  into  them  and  present  a 
much  greater  surface  to  the  passing  air  than  would 
be  supposed.  These  processes  consist  of  a  bony  founda- 
tion overlaid  with  mucous  membrane.  The  blood- 
supply  of  the  lining  of  the  nose  is  a  liberal  one  and  the 
cells  are  constantly  secreting  moisture.  As  the  air  is 


FIG.  60. — The  path  of  the  breath.  Details  of  the  upper  part  may 
be  seen  to  better  advantage  in  Fig.  38.  (n)  nasal  passage,  (p)  pharynx, 
(0  larynx,  (t)  trachea,  (6)  left  bronchus,  (e)  esophagus,  continuous 
with  the  pharynx  above. 

inhaled  it  is  affected  in  at  least  three  ways  by  its  contact 
with  the  mucous  membrane. 

In  the  first  place,  it  deposits  a  large  proportion  of 
the  dust  particles  which  it  may  have  brought  in.  As 
has  been  stated  in  Chapter  IV,  such  particles  are  removed 
by  ciliary  motion — whether  toward  the  pharynx  or  the 
nostrils  is  disputed.  In  the  second  place,  the  air  which 
is  usually  taken  in  at  a  temperature  below  that  of  the 
body  is  warmed  nearly  to  blood  heat  in  passing  through 


BREATHING  279 

the  nose.  Finally,  it  becomes  nearly  saturated  with 
water  vapor.  We  may  breathe  a  cool,  foggy  air  which 
is  already  saturated  and  still  impart  moisture  to  it 
for,  as  we  raise  its  temperature,  it  gains  in  the  capacity 
to  hold  water.  The  three  actions  named — filtration, 
warming,  and  moistening — are  probably  better  per- 
formed by  the  nose  than  they  can  be  by  the  mouth. 

The  purpose  of  the  breathing  movements  is  to  renew 
the  air  in  the  terminal  sacs  of  the  lungs.  Before  we 
can  make  clear  the  means  by  which  these  movements 
are  executed  certain  mechanical  conditions  which  pre- 
vail in  the  thorax  must  be  dealt  with.  First  of  all,  the 
fact  is  to  be  emphasized  that  the  lungs,  like  other  viscera, 
are  not  directly  attached  to  the  chest  wall.  In  dissect- 
ing a  normal  animal  we  find  no  trace  of  adhesion  between 
the  two  pleural  surfaces,  the  one  lining  the  thorax  and 
the  other  making  the  exterior  of  the  lungs.  The  only 
connection  between  either  lung  and  the  rest  of  the  body 
is  through  its  bronchus  and  the  pulmonary  vessels. 

Next  we  must  note  the  fact  that  when  an  animal  is 
dissected  the  lungs  appear  much  smaller  than  the  cavity 
which  they  completely  filled  in  life.  They  have  col- 
lapsed and  contracted  to  a  half  or  a  smaller  fraction 
of  their  former  size.  Sometimes  one  has  a  glimpse  of 
the  lung  in  the  act  of  falling  away  from  the  body  wall 
as  the  scalpel  goes  through  it.  After  their  collapse 
they  can  be  brought  back  to  their  normal  size  by  blowing 
through  a  tube  which  has  been  tied  into,  the  trachea, 
but  they  will  promptly  shrink  again  if  the  air  pressure 
is  discontinued.  Yet  the  force  needed  to  hold  them 
to  their  full  capacity  is  surprisingly  small. 

The  collapse  of  the  lungs  when  the  thorax  is  opened 
is  proof  that  they  were  always  tending  to  do  just  this 
thing — always  drawing  inward  upon  the  chest  wall. 
As  has  been  explained  in  Chapter  III,  actual  separation 
between  the  organs  and  the  body  wall  cannot  occur 
unless  air  or  liquid  is  admitted  between  them.  To  part 
them  otherwise  would  require  the  immense  force  neces- 


280  HUMAN    PHYSIOLOGY 

sary  to  overcome  the  atmospheric  pressure  and  create 
a  vacuum.  Students  of  physics  will  recall  the  Magde- 
burg Hemispheres.  A  wound  which  pierces  the  human 
thorax  may  admit  air  between  the  two  layers  of  the 
pleura  and  permit  the  collapse  of  a  lung,  giving  rise  to 
a  condition  known  as  pneumothorax.  This  can  happen 


FIG.   01. — To  suggest    the    absence   of  connection   between   the  lungs 
and  the  walls  of  the  chest.      (See  text.) 

to  one  lung  without  seriously  affecting  the  situation  of 
the  other. 

A  collapsed  lung  is  practically  useless.  It  no  longer 
follows  the  movements  of  the  chest  as  the  normal  lung 
does.  Instead  it  remains  of  a  nearly  constant  volume 
and  the  little  air  which  it  contains  is  not  renewed. 
Recovery  from  one-sided  pneumothorax  is  possible. 


BREATHING  281 

One  lung  serves  to  keep  up  respiration  for  the  time  and 
after  the  wound  is  closed  the  air  between  the  chest 
wall  and  the  contracted  lung  is  gradually  absorbed. 
In  proportion  as  it  is  removed  the  lung  approaches  its 
full  size  and  at  length  the  layers. of  the  pleura  are  once 
more  in  approximate  contact.  A  lung  may  be  reduced 
in  volume  and  in  usefulness  by  the  accumulation  of 
liquid  as  well  as  air  outside  it  and  it  is  sometimes  neces- 
sary to  withdraw  such  fluid  through  a  puncture  between 
the  ribs. 

The  Breathing  Movements. — Generally  speaking,  any 
movement  which  enlarges  the  chest  will  enlarge  the 
lungs.  The  enlargement  will  affect  chiefly  the  elastic 
terminal  sacs  which  are  much  more  susceptible  to  stretch- 
ing and  recovery  than  the  blood-vessels  or  the  bronchial 
tubes  between  these  sacs  and  the  bronchi.  When  the 
sacs  are  made  larger  air  will  press  into  them  as  into  a 
widening  bellows.  It  is  the  atmospheric  pressure  which 
drives  it  and  not  a  muscular  application  like  that  of 
swallowing.  Any  movement .  which  makes  space  for 
the  reception  of  air  is  reckoned  a  movement  of  inspira- 
tion. From  what  has  been  said  of  the  elastic  tension 
of  the  lungs  it  should  be  evident  that  these  organs  resist 
inspiration  and  assist  in  expiration.  The  one  is  made 
more  difficult  than  if  the  lungs  were  indifferent  to  it, 
while  the  other  is  facilitated.  The  condition  may  be 
likened  to  that  illustrated  by  a  screen  door  which  has  a 
spring:  it  is  harder  to  open  than  if  it  had  none,  but  it 
closes  itself. 

Elevation  of  the  ribs  is  an  important  factor  in  inspira- 
tion. It  is  not  necessary  to  analyze  the  action  of  the 
muscles  by  which  this  is  brought  about,  but  the  effect 
upon  the  dimensions  of  the  thorax  must  be  made  plain. 
There  are  twelve  pairs  of  ribs.  Each  rib  is  hinged  upon 
one  side  of  a  vertebra.  It  connects  with  the  vertebra 
at  two  points  and,  this  being  the  case,  its  movement  is 
limited  to  rotation  around  an  axis  passing  through  these 
two  points.  The  direction  of  the  axes  is  somewhat 


282 


HUMAN    PHYSIOLOGY 


different  in  the  case  of  the  upper  from  what  it  is  with 
the  lower  ribs.  We  will  consider  first  the  effect  of  raising 
the  upper  ribs  upon  the  thoracic  cavity. 

The  first  rib  is  a  short  one,   curving  with  a  small 
radius  from  its  attachment  to  the  first  dorsal  vertebra 


FIG.  62. — Above,  the  first  and  second  ribs  are  viewed  from  the  right. 
To  elevate  them  is  to  carry  forward  their  ventral  ends  and  the  breast 
bone,  deepening  the  chest. 

Below,  the  ninth  and  tenth  ribs  are  viewed  from  behind.  As  they 
are  raised  the  chief  result  is  a  lateral  spreading. 

to  reach  the  upper  end  of  the  breast  bone.  The  ventral 
ends  of  the  first  ribs  are  considerably  below  the  dorsal 
as  observed  on  the  erect  figure.  Their  axis  of  rotation 
is  around  a  line  drawn  from  right  to  left  across  the 


BREATHING  283 

vertebral  column.  If  the  conditions  are  successfully 
visualized,  it  will  be  seen  that  when  the  first  ribs  are  raised 
they  must  carry  the  breast  bone  with  them  and  at  the 
same  time  thrust  it  forward.  The  result  is  a  deepening 
of  the  upper  chest. 

As  we  follow  the  series  of  the  ribs  downward  we  find 
that  there  is  progressive  shifting  in  the  direction  of 
their  axes  of  rotation.  These  axes  never  become  quite 
dorso-ventral  (front  to  back)  but  they  tend  more  and 
more  toward  this  limit.  Our  attention  may  be  fixed 
upon  a  selected  pair,  perhaps  the  ninth  or  tenth. 
These  ribs  are  long  and  they  are  attached  to  the  lower 
part  of  the  breast  bone  only  indirectly  and  by  means  of 
flexible  cartilages.  Their  borders  are  much  lower  than 
their  articulations  with  the  spine  and  lower,  also,  than 
their  final  connection  with  the  breast  bone.  When 
these  ribs  are  raised  their  lateral  portions  are  swung 
apart  to  the  right  and  left.  Thus  the  lower  part  of  the 
chest  is  widened  more  than  it  is  deepened  in  inspiration. 

The  Diaphragm. — This  dome-shaped  partition  be- 
tween the  thorax  and  the  abdomen  has  already  been 
mentioned.  Its  central  part  is  a  flat  tendon  and  its 
borders  are  muscular,  the  general  direction  of  the  fibers 
being  radial.  It  is  one  of  the  muscles  concerning  which 
it  is  not  easy  to  distinguish  origin  and  insertion.  As 
these  terms  have  been  defined,  the  origin  of  a  muscle  is 
the  stationary  and  the  insertion  the  movable  attach- 
ment. When  the  muscle  fibers  of  the  diaphragm  are 
thrown  into  contraction  it  cannot  always  be  predicted 
that  one  part  or  another  will  be  moved.  There  are  at 
least  two  distinct  possibilities. 

We  may  picture  the  margin  as  fixed.  If  this  is  the 
case  the  curve  of  the  dome  will  be  flattened  and  the 
chest  will  gain  -in  capacity  by  the  lowering  of  its  floor. 
The  borders  of  the  diaphragm  will  be  bent  inward  and, 
with  a  more  intense  contraction,  there  will  be  an  actual 
depression  of  the  center.  The  lungs  will  be  stretched 
vertically,  their  bases  descending.  A  movement  such 


284 


HUMAN    PHYSIOLOGY 


as  this  adds  space  to  the  thoracic  cavity  and  it  would 
be  natural  to  say  that  it  must  subtract  space  from  the 
abdomen.  But  this  is  not  strictly  true.  The  abdominal 
contents  are  incompressible,  so  if  the  diaphragm  is  to 
descend  at  all  it  must  do  so  by  crowding  the  viscera  out 
of  place  and  room  has  to  be  made  for  them  by  stretching 
the  abdominal  wall.  The  abdomen  is  not  made  smaller 
but  its  shape  is  changed. 

If  the  descent  of  the  diaphragm  is  opposed,  the  chief 
effect  of  its  contraction  may  be  to  pull  its  marginal  at- 


FIG.  63. — The     diaphragm     between    empty    body    cavities.     The 
ventral  border  has  been  cut  away  to  some  extent.      (See  text.) 

tachments  upward  toward  a  stationary  center.  If  the 
ribs  could  move  freely  in  any  direction  this  would  narrow 
the  lower  part  of  the  chest.  But  since,  in  reality,  they 
cannot  rise  without  spreading,  the  movement  is  still 
inspiratory.  In  such  circumstances  the  diaphragm  has 
merely  taken  its  place  among  the  other  muscles  which 
are  adapted  to  elevate  the  ribs.  If  one  inspires  as  deeply 
as  possible  it  will  probably  be  noticed  that  up  to  a  certain 
point  the  abdominal  wall  is  pushed  steadily  out;  before 
the  movement  is  carried  to  its  limit  the  wall  has  begun 
to  fall  back.  The  protrusion  indicates  that  the  dia- 
phragm is  bearing  down  upon  the  viscera;  the  reversal 


BREATHING  285 

conies  when  the  rising  of  the  ribs  more  than  compensates 
for  this. 

Expiration. — Inspiration  requires  a  marked  exertion. 
Expiration  is  promoted  by  certain  factors  other  than 
muscular  contraction.  Mention  may  be  made  of:  (1) 
the  elastic  tension  of  the  lung  tissue,  lately  referred  to; 
(2)  the  elastic  reaction  of  the  abdominal  wall  tending, 
through  the  medium  of  the  viscera,  to  thrust  up  the  dia- 
phragm; (3)  the  elastic  reaction  of  the  cartilages  uniting 
the  ribs  with  the  breast  bone,  and  (4)  gravity,  for  when 
we  breathe  in  we  have  usually  to  raise  a  weight.  Muscles 
are  employed  to  a  variable  extent  to  compress  the  ab- 
domen and  draw  down  the  ribs.  The  muscular  activity 
may  be  strenuous  when  expiration  has  a  forced  character, 
as  in  shouting  or  blowing  forcibly. 

The  Closed  Glottis. — The  glottis  is  the  cleft  between 
the  vocal  cords  in  the  larynx  through  which  the  breath 
must  pass.  It  is  the  narrowest  segment  of  the  respiratory 
path.  It  can  be  very  securely  closed  by  the  muscles 
which  flank  it.  When  the  glottis  is  closed  the  air  in 
the  lungs  becomes  a  substantial  cushion.  We  in- 
stinctively confine  it  when  we  make  any  great  effort 
and  the  upper  part  of.  the  body  is  thereby  firmly  sup- 
ported. It  is  like  screwing  up  the  valve  without  which 
the  automobile  tire  has  no  power  to  bear  weight.  An 
expiratory  effort  with  closed  glottis  throws  pressure 
by  means  of  the  air  cushion  directly  upon  the  heart, 
hindering  its  diastole,  and  upon  the  veins,  which  may  be 
much  compressed.  The  reddening  of  the  face  is  a 
sign  of  the  backing  up  of  the  blood  along  the  veins,  and 
the  interference  with  the  circulation  may  be  great  enough 
to  cause  dizziness. 

The  Tidal  Air. — When  one  is  breathing  quietly  the 
volume  of  the  air  which  enters  or  leaves  the  nostrils  at 
a  single  breath  is  said  to  be  about  30  cubic  inches.  This 
quantity  is  called  the  tidal  air  because  of  its  regular 
coming  and  going.  At  each  breath  some  air  is  carried 
from  the  sacs  of  the  lungs  to  the  exterior  and  replaced 


286  HUMAN    PHYSIOLOGY 

with  fresh  air.  But  we  cannot  say  that  the  whole 
volume  of  the  tidal  air  is  thus  removed  and  replaced. 
We  have  to  make  allowance  for  what  is  called  the 
dead  space. 

This  term  is  applied  to  the  volume  of  air  contained 
in  the  nasal  passages,  the  pharynx,  the  trachea,  the 
bronchi,  and  the  branching  bronchial  tubes  which  lead 
to  the  microscopic  sacs.  It  amounts  to  about  10  cubic 
inches.  We  are  somewhat  handicapped  by  the  existence 
of  the  dead  space,  as  will  now  be  shown.  Suppose  that 
an  expiration  is  just  completed.  The  passages  consti- 
tuting the  dead  space  contain  air  arrested  on  its  way  out 
from  the  depths  of  the  lungs.  Inspiration  begins.  All 
the  vitiated  air  in  the  dead  space  must  be  borne  back 
again  into  the  terminal  sacs  before  any  fresh,  atmospheric 
air  can  enter  them. 

Again,  at  the  height  of  inspiration  the  dead  space  is 
occupied  by  fresh  air.  During  expiration  all  this  good 
air  must  be  rejected  before  the  impoverished  air  from  the 
actual  seat  of  the  respiratory  exchanges  begins  to  escape 
from  the  nostrils.  It  is  easy  to  see  that  a  man  breathT 
ing  through  a  long  rubber  tube  would  be  suffocated, 
because  he  would  merely  move  back  and  forth  the  same 
air  instead  of  obtaining  fresh  supplies.  We  are  un- 
avoidably hindered  by  the  actual  dead  space,  according 
to  the  same  principle,  but  the  impediment  is  not  a 
serious  one.  It  will  appear  that  the  deeper  the  breath- 
ing the  smaller  the  fraction  of  the  total  represented 
by  the  dead  space.  On  the  other  hand,  when  the  breath- 
ing becomes  shoal  from  any  cause  the  fraction  correspond- 
ing to  the  dead  space  becomes  progressively  larger.  If 
the  tidal  air  sinks  to  10  cubic  inches  there  is  little  or  no 
useful  surplus  over  the  alternating  movement  within 
the  passages. 

The  Vital  Capacity. — It  is  evident  that  we  seldom 
breathe  as  much  as  we  can.  The  ordinary  tidal  air  is 
perhaps  only  one-eighth  of  the  total  volume  which  we 
can  take  in  after  a  forced  expiration  or  expire  after  the 


BREATHING  287 

deepest  inspiration.  The  maximum  volume  of  a  single 
breath,  commonly  about  250  cubic  inches,  is  called  the 
vital  capacity.  It  is  often  spoken  of  as  the  capacity  of 
the  thorax,  but  this  is  evidently  inaccurate.  After  the 
most  intense  expiratory  effort  there  is  still  air  in  the  lungs 
which  we  cannot  crowd  out.  This  is  the  so-called  residual 
air  and  it  is  said  to  amount  to  some  60  cubic  inches. 
It  is  our  habit  to  breathe  with  the  chest  in  an  intermediate 
position,  about  half  way  between  extreme  distention  and 
severest  compression. 

The  Changes  in  the  Respired  Air. — These  are  partly 
physical  and  partly  chemical.  In  the  former  class  we 
note  the  warming  and  saturation  of  the  air  already 
mentioned.  As  much  as  a  pint  of  water  may  be  re- 
moved from  the  body  by  the  breath  in  the  course  of 
twenty-four  hours.  We  ought  not  to  say  that  this  is 
from  the  lungs  for  it  comes  chiefly  from  the  nasal  lining. 
The  air  is  warmed  and  moistened  on  its  way  in  and  the 
water  from  the  nose  is  carried  into  the  lungs  to  be  brought 
back  again  to  the  exterior. 

The  chemical  changes  call  for  a  fuller  statement. 
We  anticipate  that  expired  air  will  contain  less  oxygen 
than  standard  atmospheric  air  and  more  carbon  dioxid. 
The  actual  alteration  is  less  extensive  than  might  be  ex- 
pected. The  outside  air  contains  nearly  21  per  cent, 
of  oxygen;  the  percentage  in  expired  air  may  be  reduced 
to  16.  The  quantity  of  carbon  dioxid  in  the  atmosphere 
is  insignificant — about  0.04  per  cent.  The  amount  in 
expired  air  may  be  4  per  cent,  or  a  little  more.  All  such 
figures  are  naturally  subject  to  variation.  But  we  have 
learned  to  anticipate  that  a  disappearance  of  5  volumes 
of  oxygen  will  be  simultaneous  with  the  discharge  of  4 
or  4.5  volumes  of  carbon  dioxid. 

The  fact  that  the  carbon  dioxid  discharged  is  normally 
a  little  less  than  the  oxygen  consumed  has  an  interesting 
significance.  If  all  the  oxygen  taken  into  the  blood- 
stream went  to  form  carbon  dioxid  the  volume  of  this 
gas  would  just  equal  that  of  the  oxygen  used  to  generate 


288  HUMAN    PHYSIOLOGY 

it.  But  a  moderate  amount  of  water  is  constantly 
formed  as  a  second  respiratory  product.  The  oxygen 
which  goes  to  form  water,  of  course,  fails  to  appear  in 
combination  with  carbon.  Later  on  we  shall  find  that 
the  proportion  between  the  oxygen  absorbed  and  the 
carbon  dioxid  evolved  by  an  animal  furnishes  valuable 
information  to  the  student  of  metabolism. 

If  the  expired  air  contain  16  per  cent,  of  oxygen  and 
4  per  cent,  of  carbon  dioxid,  as  we  have  assumed,  we 
must  still  bear  in  mind  that  the  first  portion  of  each 
expiration  is  from  the  dead  space  and  hence  almost 
like  fresh  air.  It  follows  that  the  last  part  of  each  ex- 
piration is  lower  in  oxygen  and  higher  in  carbon  dioxid 
than  the  average  of  the  whole  and  that  it  is  a  fair  sample 
of  the  air  in  the  terminal  sacs  of  the  lungs.  Such  air 
may  contain  nearly  6  per  cent,  of  carbon  dioxid  and 
something  like  14  per  cent,  of  oxygen.  It  is  the  alveolar 
air  and  it  is  this  which  we  must  consider  as  standing  in 
relation  to  the  blood  in  the  pulmonary  capillaries. 

The  nitrogen  which  makes  79  per  cent,  of  the  atmos- 
pheric air  is  not  believed  to  be  drawn  upon  or  added 
to  by  the  blood.  This  is  the  case,  at  any  rate,  in  the 
long  run.  When  there  is  a  rise  of  barometric  pressure 
there  is  doubtless  some  absorption  of  additional  nitrogen 
by  the  blood  and  when  the  pressure  falls  again  there  is 
some  yielding  up  of  nitrogen.  But  this  is  an  accidental 
matter  and  not  related  to  the  life-processes. 

Whether  anything  of  an  organic  nature  passes  out 
with  the  expired  air  is  a  question  that  has  been  much 
discussed  and  perhaps  not  fully  decided.  It  used  to 
be  asserted  that  the  breath  carries  from  the  body  com- 
pounds of  a  volatile  and  poisonous  character.  This  has 
been  difficult  to  establish  by  experiment.  Of  course  the 
breath  may  have  an  odor  and  this  indicates  the  presence 
of  some  substance  but  not  in  quantity  to  be  measured 
by  other  tests  than  the  olfactory.  Odor,  when  present, 
is  not  likely  to  be  due  to  anything  coming  from  the  lungs 


BREATHING  289 

but  to  remains  of  food,  decaying  teeth,  and  degenerating 
tissue  about  the  tonsils  and  pharynx. 

Ideals  of  Ventilation. — This  is  a  subject  which  may 
be  touched  upon  here.  We  all  recognize  that  rooms 
in  which  the  air  is  not  freely  changed  become  " stuffy" 
and  depressing  to  work  in.  Just  what  constitutes  "bad 
air"  as  noticed  in  such  situations  has  been  differently 
interpreted  by  writers  of  different  periods.  The  simple 
idea  that  air  in  close  rooms  lacks  oxygen  or  is  over- 
charged with  carbon  dioxid  cannot  be  maintained. 
The  existence  in  it  of  volatile  poisons  is,  as  we  have  said, 
hard  to  establish.  An  odor  may  have  a  considerable 
effect  upon  the  human  nervous  system  and  it  is  to  be 
noted  that  odors  in  ill-ventilated  places  are  not  due 
simply  to  the  breath  but  proceed  from  skins,  clothing, 
lights,  and  many  other  sources. 

According  to  the  opinion  now  generally  held  the  chief 
trouble  with  an  atmosphere  that  is  regarded  as  stuffy 
or  close  is  that  it  is  overheating  to  the  skin.  The  sur- 
face of  the  human  body  is  kept  comfortably  cool  under 
favorable  conditions  by  the  air  currents  which  pass  over 
it  and  by  the  evaporation  of  perspiration.  The  former 
are  an  aid  to  the  latter.  In  a  confined  space  there  is  a 
lack  of  movement  in  the  air  and  it  tends  also  to  become 
warm  and  humid.  Moisture  is  not  taken  promptly 
from  the  skin  and  its  temperature  rises.  A  vasomotor 
reflex  is  encouraged  by  which  the  cutaneous  vessels  are 
dilated  and,  with  the  increased  blood-flow,  the  unpleasant 
warmth  is  made  more  pronounced.  There  is  likely  to 
be  some  reduction  of  the  general  blood-pressure,  leading 
to  drowsiness  or,  at  least,  a  feeling  of  inertia. 

If  these  views  are  correct,  the  most  effective  precau- 
tions that  can  be  taken  to  secure  comfort  in  a  room  are 
to  keep  it  cool  and  to  have  some  circulation  of  the  air. 
Our  fear  of  strong  drafts  may  be  a  wholesome  one  but 
we  should  also  avoid  stagnation.  It  has  been  shown 
that  starting  an  electric  fan  in  a  close  room  may  relieve 
an  almost  intolerable  condition.  It  does  not  improve 

19 


290  HUMAN    PHYSIOLOGY 

the  air  chemically  but  it  favors  the  removal  of  heat  from 
the  bodies  of  the  inmates  and  braces  up  their  vasomotor 
systems.  An  English  writer,  adopting  a  novel  but  sug- 
gestive point  of  view,  has  said  the  real  difficulty  with  a 
stuffy  room  is  that  there  is  a  lack  of  stimulation  for  the 
nervous  system.  One  becomes  relaxed  and  indolent 
because  the  receptors  are  not  being  played  upon  as  they 
would  be  by  a  breeze  or  even  by  a  decidedly  chilling 
air  .This  is  consistent  with  the  recognized  fact  that  the 
most  comfortable  climates  do  not  make  for  the  greatest 
efficiency. 

We  may  ask  in  passing  what  are  the  causes  of  death 
when  people  are  huddled  in  narrow  quarters.  The  classic 
illustration  is  that  of  the  Black  Hole  of  Calcutta.  About 
150  English  soldiers  were  confined  through  a  tropic  night 
in  a  room  20  feet  square.  There  were  but  two  windows, 
both  on  the  same  side  of  the  room.  Only  twenty-three 
men  survived.  Perhaps  in  a  case  as  extreme  as  this 
actual  shortage  of  oxygen  and  damaging  excess  of  carbon 
dioxid  might  have  been  realized.  There  might  also  have 
been  definite  poisoning  from  volatile  excretions.  But 
another  factor  which  we  cannot  overlook  must  have  been 
the  collective  rise  of  temperature  resulting  from  the 
crowding  together  of  so  many  bodies.  This  was  made 
more  acute  by  the  struggle  which  went  on  among  the 
agonized  prisoners. 


CHAPTER  XXI 
RESPIRATION  (Continued) 

Once  or  twice  each  minute  all  the  blood  in  the  body 
passes  through  the  capillaries  of  the  lungs.  As  it  enters 
from  the  right  side  of  the  heart  it  is  reckoned  venous; 
it  goes  on  to  the  left  auricle  and  ventricle  arterial  blood. 
It  has  been  stated  before  that  arterial  blood  contains 
about  all  the  oxygen  which  can  be  attached  to  the 
hemoglobin  of  the  corpuscles,  while  average  venous  blood 
is  not  by  any  means  devoid  of  oxygen.  It  is  likely  to 
retain  upward  of  half  the  amount  of  the  gas  originally 
present.  The  facts  concerning  the  carbon  dioxid  are 
perhaps  rather  unexpected. 

Carbon  dioxid  is  the  most  abundant  gas  even  in  arterial 
blood.  So  much  of  it  is  constantly  present  there  that 
the  increase  in  venous  blood  seems  rather  slight.  The 
quantity  obtainable  from  100  volumes  of  arterial  blood 
may  be  in  the  vicinity  of  38  volumes.  From  100  volumes 
of  venous  blood  about  45  volumes  of  carbon  dioxid  may 
probably  be  removed.  (Blood  exposed  to  a  vacuum  will 
froth  and  rapidly  give  off  all  the  mixed  gases  which  it 
has  been  holding.)  The  oxygen  in  100  volumes  of 
arterial  blood  may  be  20  volumes;  in  venous  blood 
about  12. 

The  time  during  which  any  single  corpuscle  or  any 
portion  of  plasma  is  in  a  position  for  respiratory  ex- 
change is  only  a  second  or  two.  It  is  shorter  when  the 
circulation  is  accelerated  to  support  muscular  activity. 
Yet  it  appears  to  suffice  for  the  purpose.  There  is  some 
uncertainty  at  the  present  time  whether  the  delicate 
cells  through  which  the  transfer  of  oxygen  and  carbon 
dioxid  must  take  place  act  like  an  inert  membrane  or 
apply  energy  of  their  own  evolution  to  promote  the 

291 


292  HUMAN    PHYSIOLOGY 

process.  The  common  teaching  has  been  that  they 
are  indifferent  to  what  is  going  on  as  if  they  were  lifeless 
structures  but  we  may  be  obliged  to  credit  them  with 
a  definite  participation  in  the  performance. 

Internal  Respiration. — Emphasis  has  been  placed 
on  the  preliminary  character  of  the  exchanges  occurring 
in  the  lungs.  It  has  been  insisted  that  respiration  in 
the  best  .sense  of  the  term  is  a  function  of  the  tissues  at 
large  in  proportion  as  they  share  in  the  general  metabo- 
lism. We  sometimes  distinguish  this  genuine  biologic 
respiration  as  internal,  contrasting  it  with  the  pul- 
monary give  and  take  which  we  call  external  respira- 
tion. From  this  point  of  view  external  respiration  makes 
venous  blood  arterial  and  internal  respiration  makes 
arterial  blood  venous. 

While  the  blood  is  flowing  slowly  through  the  systemic 
capillaries,  it  is  separated  from  the  surrounding  lymph 
by  a  partition  of  the  most  delicate  description.  The 
living  cells,  contractile  or  glandular  or  other,  are  in 
contact  with  the  lymph.  They  keep  the  oxygen  con- 
tent of  the  lymph  at  a  low  level  but  do  not  absolutely 
exhaust  it  because  its  very  poverty  in  this  gas  is  an 
invitation  to  bring  in  fresh  supplies  from  the  passing 
blood.  The  immediate  source  is  the  plasma  but  this 
has  a  very  limited  oxygen  capacity  and  when  drawn 
upon  it  takes  more  from  the  corpuscles,  turning  oxyhemo- 
globin  into  reduced  hemoglobin. 

Meanwhile  the  active  cells  are  thrusting  the  carbon 
dioxid  which  they  produce  into  the  lymph  and  raising 
the  concentration  of  the  waste  in  this  fluid  to  a  point 
favorable  to  its  transfer  into  the  blood.  It  enters  the 
plasma  and  is  carried  mainly  by  that  part  of  the  blood 
in  various  obscure  combinations.  A  moderate  share  is 
united  with  the  hemoglobin  of  the  corpuscles  which  can 
contain  carbon  dioxid  without  losing  their  power  to 
hold  oxygen. 

Carbon  Monoxid. — This  is  a  gas  which  is  not  to  be 
confused  with  carbon  dioxid.  It  is  formed  when  carbon 


HESPIRATION  293 

is  burned  with  an  insufficient  supply  of  oxygen  gas,  as 
when  fresh  coal  is  placed  upon  the  top  of  a  furnace  fire. 
It  does  not  arise  in  the  living  body  and  its  interest  in  this 
connection  comes  from  the  danger  of  inhaling  it.  We 
may  be  subjected  to  its  effects  when  it  has  reached  us 
from  fires  or  when  illuminating  gas  escapes  into  our 
rooms.  The  common  form  known  as  "water  gas"  con- 
tains it  in  great  quantity.  It  is  very  poisonous  and 
for  a  simple  reason:  it  deprives  the  blood  of  its  oxygen- 
carrying  capacity. 

Carbon  monoxid  does  this  by  replacing  oxygen  in  the 
oxyhemoglobin  molecule.  The  unnatural  compound 
formed  is  a  stable  one  and  circulates  with  little  tendency 
to  release  the  carbon  monoxid  which  is  locked  in  it. 
Just  so  far  as  the  corpuscles  become  engaged  in  carry- 
ing this  useless  gas  they  are  incapacitated  for  their 
essential  service.  If  the  substitution  becomes  extensive 
enough  the  lack  of  oxygen  becomes  fatal,  the  heart  and 
the  nervous  system  failing  for  want  of  it  much  as  though 
the  victim  were  bleeding  to  death.  The  blood,  when 
saturated  with  carbon  monoxid,  has  a  light  color  de- 
scribed as  cherry  red  and  does  not  darken  on  standing 
as  normal  blood  does. 

Breathing  Pure  Oxygen. — The  impression  is  common 
that  the  experience  of  breathing  pure  oxygen  «is  a  highly 
exhilarating  one.  This  is  not  really  the  case  unless  an 
element  of  suggestion  secures  the  anticipated  result. 
If  a  man  does  not  know  that  he  has  been  given  the  oxygen 
to  breathe  he  is  not  likely  to  report  that  the  experience 
is  at  all  peculiar.  Nevertheless  oxygen  has  often  seemed 
to  be  of  the  utmost  value  in  critical  pulmonary  disease. 
We  must  try  to  reconcile  the  negative  reaction  of  one 
who  breathes  it  in  health  and  the  marked  relief  of  the 
pneumonia  patient.  A  simple  statement  may  explain 
the  difference:  oxygen  does  not  alter  the  composition  of 
normal  blood  in  a  way  that  will  much  affect  the  organism 
but  it  is  potent  to  restore  to  the  standard  blood  which 
may  have  fallen  below  in  illness. 


294  HUMAN    PHYSIOLOGY 

It  has  lately  been  shown  that  oxygen  is  distinctly 
helpful  to  athletes  in  certain  trials  of  strength  and  speed. 
This  is  best  evidenced  by  middle-distance  runners. 
They  do  not  greatly  improve  their  time  under  its  in- 
fluence but  they  finish  with  a  minimum  of  distress.  How 
can  we  find  any  condition  common  to  the  runner  in  the 
height  of  condition  and  the  sick  man  fighting  for  his 
life?  This  is  easier  than  might  be  supposed.  Both  are 
tending  to  suffer  from  a  falling  of  the  blood  below  its 
normal  composition  as  regards  oxygen.  In  the  case  of 
the  athlete  this  is  due  to  the  great  demands  which  his 
muscles  are  making  upon  the  blood.  In  the  case  of  the 
sick  man  it  is  not  huge  consumption  of  oxygen  but 
restricted  supply  which  is  responsible  for  a  somewhat 
similar  chemical  disturbance. 

The  healthy,  resting  man,  if  he  breathes  oxygen,  adds 
something  to  the  amount  circulating  in  his  system.  But 
his  tissues  were  already  having  offered  to  them  so  liberal 
a  supply  that  they  returned  much  unused  in  the  venous 
blood.  No  important  results  will  follow  if  they  are 
offered  more.  The  tissues  will  not  burn  under  "  forced 
draft."  This  is  an  important  difference  between  the 
relation  of  oxygen  to  living  things  and  to  fires. 

Compressed  Air. — In  many  engineering  operations, 
especially  in  constructing  tunnels  or  laying  foundations 
far  below  water,  it  is  necessary  to  fill  the  spaces  in 
which  the  men  are  working  with  highly  compressed 
air.  The  atmospheric  pressure  is  about  15  pounds  to 
the  square  inch.  At  34  feet  below  water  twice  this 
pressure  must  be  provided  to  keep  the  water  pressure 
balanced.  At  68  feet  the  air  pressure  must  be  three 
times  the  atmospheric,  three  atmospheres,  as  we  ex- 
press it.  Such  works  have  often  been  carried  on  at 
greater  depths  than  this  and  pressures  of  four  or  more 
atmospheres  have  been  employed.  Divers  can  descend 
for  short  periods  to  levels  at  which  the  air  pumped  into 
their  helmets  has  to  be  compressed  to  six  or  seven  times 
the  normal  density. 


RESPIRATION  295 

It  has  long  been  known  that  compressed  air  has  its 
dangers.  Only  slight  disturbances  are  experienced  on 
being  introduced  into  it,  but  serious  or  even  fatal  effects 
may  follow  emergence  from  it.  The  symptoms  range 
all  the  way  from  dizziness  and  muscular  pain  to  apo- 
plectic death.  Experience  has  shown  that  the  workmen 
must  spend  some  time  in  atmospheres  of  intermediate 
pressures  when  coming  out  of  deep  workings.  Technic- 
ally speaking,  they  must  be  decompressed  by  stages. 
The  various  troubles  manifested  in  cases  of  too  rapid 
decompression  are  spoken  of  collectively  as  caisson- 
sickness,  divers'  palsy,  or  compressed-air  illness. 

The  earlier  theories  advanced  to  account  for  these 
difficulties  were  elaborate  and  confusing.  The  actual 
basis  of  caisson-sickness  has  been  found  to  be  exceedingly 
simple.  What  happens  in  the  tissues  of  a  man  who 
comes  too  hastily  into  the  open  after  a  stay  in  compressed 
air  is  just  what  happens  when  a  bottle  of  charged  water 
is  uncorked;  there  is  a  formation  of  bubbles,  an  efferves- 
cence. During  the  previous  period  both  oxygen  and 
nitrogen  have  been  absorbed  in  unusual  amounts,  first 
by  the  blood  and  secondarily  by  the  tissues.  It  is  the 
nitrogen  in  this  case  which  threatens  damage  by  separat- 
ing itself  in  the  form  of  minute  bubbles  when  the  pressure 
that  held  it  in  solution  is  removed.  One  can  under- 
stand how  the  most  varied  effects  may  follow  the 
development  of  bubbles  in  one  place  or  another. 

Nitrogen  bubbles  forming  in  the  nervous  system 
may  so  disorganize  its  structure  as  to  work  a  mortal 
injury,  perhaps  by  playing  havoc  with  the  respiratory 
center.  Their  presence  in  muscles  and  joints  may  result 
in  nothing  more  serious  than  pain  and  stiffness.  The 
conditions  will  be  slow  to  pass  off  for  the  cells  of  the 
body  have  little  affinity  for  nitrogen  and,  in  fact,  it  is 
hard  to  account  for  its  final  disappearance.  A  certain 
rigidity  of  the  joints  is  so  often  experienced  that  it  has 
given  to  caisson-sickness  the  colloquial  name  of  "the 
bends"  commonly  applied  to  it  by  workmen. 


296  HUMAN    PHYSIOLOGY 

Carbon  Dioxid  and  the  Respiratory  Center. — A  brief 
statement  was  made  in  Chapter  VIII  respecting  the 
control  exercised  by  the  'medulla  over  the  breathing 
movements.  Attention  was  there  drawn  to  the  fact  that 
the  center  from  which  the  impulses  issue  to  command 
the  respiratory  muscles  is  regulated  in  its  action  largely 
by  the  concentration  of  carbon  dioxid  in  the  circulating 
blood.  The  changes  in  the  quantity  of  this  gas  are 
very  positive  sources  of  influence  upon  the  neurons  of 
the  center.  To  make  this  clear  it  will  be  well,  first  of 
all,  to  show  that  the  composition  of  the  air  in  the  lungs 
has  little  if  any  effect  on  the  character  of  the  breathing. 
The  center  is  responsive  to  the  chemical  state  of  its  im- 
mediate environment  rather  than  to  that  which  pre- 
vails in  the  air-sacs.  A  striking  experiment  shows  this 
decisively. 

Two  rabbits  are  anesthetized  and  placed  side  by  side. 
By  an  operation  of  some  delicacy  connections  are  made 
which  lead  the  blood  from  the  arterial  system  of  rabbit 
A  to  the  head  of  rabbit  B.  Similarly  the  arteries  of 
rabbit  B  are  made  to  supply  the  head  of  rabbit  A.  If 
the  trachea  of  rabbit  A  is  now  obstructed  it  is  the  second 
animal  which  shows  the  labored  breathing  movements 
of  asphyxia.  The  air  in  the  lungs  of  rabbit  A  is  falling 
off  in  oxygen  and  gaining  in  carbon  dioxid  but  this  does 
not  stimulate  the  nervous  mechanisms  so  long  as  the 
standard  blood  of  rabbit  B  is  flowing  through  the  vessels 
of  the  brain. 

When  the  composition  of  the  blood  tends  to  be  altered 
either  because  of  restricted  breathing  or  excessive 
muscular  activity  we  may  expect  a  simultaneous  rise 
in  the  carbon  dioxid  of  the  blood  and  a  fall  in  its  oxygen. 
How  do  we  know  which  of  these  changes  stirs  the  re- 
spiratory center  to  compensatory  action?  It  is  possi- 
ble for  purposes  of  experiment  to  separate  the  two.  A 
trial  may  be  made  upon  the  human  subject  without  un- 
due hardship.  A  man  may  breathe  in  and  out  of  a 
silk  bag,  his  breath  passing  back  and  forth  through  a 


RESPIRATION  297 

vessel  containing  alkali.  Exposure  of  the  air  to  the 
alkali  will  remove  the  carbon  dioxid,  but  there  is  no 
provision  here  for  a  restoration  of  the  consumed  oxygen. 

The  subject  of  such  an  experiment  as  this  will  breathe 
for  a  long  time  without  any  exaggerated  efforts  and 
without  feeling  distressed.  He  may  become  quite  blue 
and  will  perhaps  lose  consciousness  if  the  trial  is  not  in- 
terrupted. It  appears  that  lack  of  oxygen  does  not 
strongly  excite  the  respiratory  center  even  when  it  is 
threatening  to  prove  fatal.  The  complementary  ex- 
periment consists  in  giving  a  man  a  mixture  of  carbon 
dioxid  and  oxygen  to  breathe.  In  this  case  labored 
breathing  will  result,  even  though  the  mixture  be  95 
per  cent,  of  oxygen  to  5  per  cent,  of  carbon  dioxid.  The 
blood  is  perfectly  arterialized  so  far  as  oxygen  is  concerned 
but  a  very  moderate  increase  in  its  carbon  dioxid,  due 
to  the  difficulty  of  shaking  off  the  waste  under  such 
circumstances,  is  enough  to  act  powerfully  on  the  center. 

Forced  Breathing. — If  a  person  sets  himself  at  work 
to  breathe  as  fast  and  deeply  as  he  can  he  will  soon  be 
uncomfortable.  He  may  be  dizzy  or  faint.  The  in- 
clination to  stop  forcing  the  breathing  becomes  hard  to 
resist.  In  fact  it  is  almost  as  hard  to  overbreathe  as 
to  underbreathe,  if  these  expressions  may  be  allowed. 
The  effects  of  breathing  a  greater  volume  of  air  than  that 
needed  to  meet  current  needs  are  attributed  to  a  reduc- 
tion of  the  carbon  dioxid  in  the  blood.  The  condition 
in  which  the  carbon  dioxid  of  the  arterial  blood  is  below 
the  standard  is  known  as  acapnia. 

In  view  of  the  fact  that  carbon  dioxid  is  the  foremost 
of  animal  waste-products  it  might  be  supposed  that  it 
could  not  be  too  thoroughly  removed  for  the  advantage 
of  the  organism.  But  our  natural  impression  has  proved 
to  be  wrong.  When  the  lungs  are  rapidly  ventilated 
the  alveolar  air  comes  to  have  an  unusual  composition. 
It  approximates  to  the  fresh  outside  air.  The  increase 
in  oxygen,  perhaps  from  14  to  18  per  cent.,  has  little  effect 
on  the  blood.  The  simultaneous  decrease  in  the  alveolar 


298  HUMAN    PHYSIOLOGY 

carbon  dioxid  from  something  like  6  per  cent,  to  half  as 
much  permits  a  corresponding  reduction  in  the  carbon 
dioxid  of  the  blood. 

It  seems  to  be  established  that  a  certain  concentration 
of  carbon  dioxid  in  the  blood  and  tissues  is  necessary  to 
normal  reaction.  We  may  think  of  the  gas  as  having 
a  mild  stimulating  effect  upon  the  nerve  centers  and  as- 
sume that  the  want  of  it  results  in  a  paralysis  of  some  of 
these  cell-groups.  The  case  of  the  respiratory  center 
seems  especially  clear.  We  have  seen  that  this  center 
is  stimulated  to  increased  activity  when  the  local  con- 
centration of  carbon  dioxid  is  raised.  Conversely  it  may 
cease  to  act  for  a  time  if  the  carbon  dioxid  in  its  vicinity 
is  cut  down.  Thus  acapnia  produces  a  suspension  of 
breathing — apnea,  as  we  call  it.  When  the  breathing 
is  at  a  standstill  an  increase  of  carbon  dioxid  is  to  be 
expected  and  this  will  lead  to  a  resumption  of  breath- 
ing within  a  short  time.  Yet  it  is  possible  that  the 
stimulus  to  resume  breathing  may  not  be  furnished 
until  the  shortage  of  oxygen  has  wrought  irreparable 
damage. 

High  Altitudes. — The  barometric  pressure  is  lowered 
progressively  as  one  ascends  from  the  sea-level.  At  a 
height  of  15,000  feet,  corresponding  to  that  of  the  Alps 
and  the  Rockies,  it  is  reduced  to  about  one-half  the  coast 
standard.  This  reduction  may  be  thought  of  as  affect- 
ing both  the  nitrogen  and  the  oxygen  gases.  (The  share 
of  the  total  pressure  which  can  be  referred  to  any  gas 
in  a  mixture  is  said  to  be  the  " partial  pressure"  of  that 
gas.)  No  distinct  results  follow  the  gradual  diminution 
of  the  partial  pressure  of  the  nitrogen  as  one  goes  up  a 
mountain  but  the  lessening  of  the  oxygen  pressure  makes 
itself  felt. 

If  the  oxygen  in  the  blood  were  taken  up  according  to 
the  laws  of  physical  solution,  halving  the  pressure 
would  also  halve  the  quantity  of  the  gas  absorbed.  But 
we  have  seen  that  the  union  of  oxygen  with  hemoglobin 
is  a  chemical  one  and  it  is  rapidly  accomplished  even 


RESPIRATION  299 

when  the  pressure  of  the  oxygen  is  far  less  than  usual. 
Nevertheless  a  sufficiently  radical  reduction  of  the 
oxygen  pressure  tends  to  lessen  the  amount  received  by 
the  blood  and  the  want  begins  to  be  felt  in  the  nervous 
system.  Deficiency  of  oxygen  supply  to  active  tissues 
has  one  outstanding  result:  it  leads  to  an  accumulation 
of  acid,  to  acidosis  as  we  say. 

The  acid  which  begins  to  gather  when  the  oxygen 
supply  is  short  is  chiefly  lactic.  We  have  already  men- 
tioned this  among  the  fatigue  substances  arising  in  work- 
ing muscle.  It  can  be  rapidly  and  completely  removed 
if  oxygen  is  freely  obtainable.  If  its  formation  con- 
tinues it  acts  as  a  poison.  Mountain  sickness,  the 
combination  of  ill  effects  experienced  by  those  who 
go  to  high  altitudes  for  the  first  time,  is  believed  to  be 
due  mostly  to  lactic  acid  formation.  As  observed  on 
Pike's  Peak  in  subjects  who  are  not  exerting  them- 
selves it  is  said  to  begin  with  a  period  of  nervous  irrita- 
bility. This  is  succeeded  by  nausea  and  vertigo  and 
these  symptoms  in  their  turn  by  persistent  and  severe 
headache.  The  sequence  is  like  that  exhibited  in 
connection  with  alcoholic  indulgence. 

While  these  manifestations  are  best  explained  as  signs 
of  acidosis  it  has  been  suggested  by  writers  dealing  with 
this  subject  that  a  certain  degree  of  acapnia  is  to  be 
expected.  In  a  rarefied  atmosphere  the  breathing  must 
be  deepened  to  keep  an  adequate  oxygen  pressure  in  the 
alveolar  air.  There  is  no  difficulty,  however,  in  working 
off  the  carbon  dioxid  which  leaves  the  blood  quite  as 
freely  as  at  the  sea-level.  An  awkward  dilemma  arises: 
the  breathing  must  be  forced  to  secure  enough  oxygen, 
yet  it  cannot  be  forced  without  establishing  some 
measure  of  acapnia. 

Acclimatization. — It  would  be  interesting  to  know  all 
the  changes  which  take  place  in  the  human  system  when 
it  is  becoming  adapted  to  life  at  an  altitude  previously 
unwonted.  One  of  the  earliest  to  be  recognized  was 
a  marked  increase  in  the  number  of  the  red  corpuscles. 


300  HUMAN    PHYSIOLOGY 

If  the  charge  of  oxygen  borne  by  a  single  corpuscle 
is  smaller  at  the  new  habitation  than  it  was  at  the  old 
there  will  clearly  be  a  gain  in  adding  to  the  number  of 
the  carriers.  But  we  shall  probably  be  wrong  if  we  make 
the  increase  in  the  number  of  corpuscles  the  principal 
feature  of  the  adaptation. 

Students  commonly  fail  to  appreciate  that  the  main- 
tenance of  respiration  calls  for  an  efficient  and  economic- 
ally directed  circulation  just  as  urgently  as  for  sound 
lungs  and  trained  breathing  muscles.  A  person  will 
be  breathless  if  his  circulation  is  not  equal  to  its  task, 
regardless  of  his  success  in  ventilating  the  lungs.  When 
exercise  is  undertaken  there  must  be  a  coordination  be- 
tween the  increase  of  the  breathing  and  the  accelera- 
tion of  the  blood-flow.  It  is  hard  to  see  that  anything 
is  gained  by  the  first  unless  it  is  supported  by  the  second. 
In  becoming  acclimated  to  life  at  a  high  altitude  one 
may  be  supposed  to  acquire  several  advantages.  The 
breathing  muscles  become  stronger  and  their  energy 
is  better  directed.  The  heart  is  trained.  The  vaso- 
motor  adjustments  give  the  best  possible  direction  to 
the  blood-stream.  There  are  more  corpuscles  available 
as  noted  above. 

Some  investigators  hold  that  after  giving  due  weight 
to  all  these  possibilities  it  is  still  necessary  to  add  an- 
other to  the  list.  This  remarkable  adaptation,  they 
claim,  is  a  local  change  in  the  nature  of  the  cells  lining 
the  air-sacs  of  the  lungs.  It  is  asserted  that  there  is 
evidence  to  show  that  these  epithelial  cells  gain  a  power 
which  they  did  not  have  before,  the  ability  to  promote 
the  transfer  of  oxygen  from  the  air  to  the  blood  by  some 
application  of  their  own  energy.  It  is  hard  to  tell 
whether  this  view  will  be  widely  accepted.  It  is  a  sound 
principle  to  make  the  most  of  the  simpler  explanations 
that  offer  themselves  before  resorting  to  those  that  are 
more  complicated.  We  may  find,  however,  that  an 
active  intervention  on  the  part  of  these  cells  must  some- 
times be  assumed. 


RESPIRATION  301 

The  Hygiene  of  Breathing. — A  great  deal  is  written 
by  popular  teachers  of  hygiene  in  support  of  habitual 
deep  breathing  and  the  practice  of  special  breathing 
exercises.  It  is  certainly  desirable  to  be  able  to  breathe 
deeply,  to  have  a  large  vital  capacity.  It  is  not  well 
to  fall  into  indolent  habits  which  lead  to  the  disuse  of 
many  of  the  muscles  adapted  to  help  in  inspiration  nor 
to  fail  to  use  all  parts  of  the  lungs  at  times.  The  re- 
sistance of  the  lung  tissue  to  tuberculosis  and  other 
diseases  is  undoubtedly  increased  when  it  is  well  subjected 
to  mechanical  movements. 

On  the  other  hand,  contrary  to  the  usual  instruction, 
the  best  breathing  exercises  are  those  which  are  taken 
involuntarily  as  a  part  of  general  muscular  activity. 
We  have  seen  that  it  is  possible  to  derange  the  composi- 
tion of  the  blood  by  overbreathing.  We  cannot  ^easily 
overbreathe  when  a  great  respiratory  requirement  has 
to  be  met,  as  in  running  or  playing  tennis.  We  may 
breathe  deeply  without  tending  to  produce  acapnia  if 
we  slow  the  rhythm  of  the  movements  at  the  same  time. 
Singing  is  an  excellent  exercise,  demanding  as  it  does  the 
quick,  strong  intake  of  the  breath  and  the  prolonged, 
carefully  regulated  expiration. 

It  has  often  been  noted  that  breathing  exercises  have 
well-marked  mental  effects.  Too  much  has  probably 
been  made  of  this  fact  and  undesirable  results  have  often 
been  spoken  of  as  though  they  were  to  be  sought  for. 
Acapnia  produces  mental  confusion  and  the  state  has 
been  described  as  one  of  exaltation.  Oriental  religion- 
ists have  urged  this  claim  upon  their  western  disciples. 
From  a  scientific  standpoint  it  appears  morbid  and 
dangerous.  The  mild  emotional  reaction  of  the  singer 
is  as  much  as  can  safely  be  recommended. 

While  there  is  no  good  reason  to  advise  people  to 
breathe  more  deeply  than  their  inclination  suggests, 
it  is  desirable  to  breathe  from  a  high  base-level — that  is, 
with  the  chest  well  rounded  even  in  expiration.  This 
habit,  if  not  foolishly  exaggerated,  helps  to  maintain 


302  HUMAN    PHYSIOLOGY 

good  posture,  to  give  the  heart  a  clear  space  in  which 
to  work  and  a  desirable  traction  outward  to  assist  its 
diastole.  It  also  widens  the  cross-section  of  the  pul- 
monary vessels  so  that  the  blood  need  not  be  hurried 
through  them,  lengthens  its  time  of  exposure  to  the 
alveolar  air,  and  lightens  the  labor  of  the  right  ventricle. 
There  are  various  involved  relations  of  a  mechanical 
sort  between  the  breathing  movements  and  the  blood- 
flow  which  are  too  difficult  to  discuss  in  a  work  like  the 
present. 


CHAPTER  XXII 
METABOLISM 

The  word  metabolism  has  been  used  once  or  twice 
before  this  time  to  signify  the  sum  of  the  chemical 
changes  taking  place  in  the  body.  The  German  equiva- 
lent is  very  significant:  it  is  the  word  Stoffwechsel,  which 
means  "  transformation  of  matter."  We  exclude  from 
the  subject  the  digestive  reactions  which  go  on  in  the 
alimentary  tract;  strictly  speaking  these  do  not  occur  in 
the  body  but  near  its  surface  where  it  is  infolded.  Our 
starting  point  must  be  the  absorption  of  the  digestive 
products,  the  final  topic  of  Chapter  XV.  Most  of  what 
we  have  now  to  discuss  might  be  entitled  The  History 
of  the  Food  after  Absorption. 

Metabolism  of  Fat. — The  story  is  most  simple  in  the 
case  of  the  fats.  It  will  be  recollected  that  these  com- 
pounds are  decomposed  in  the  intestine  but  reconstructed, 
it  appears,  in  the  very  act  of  passing  through  the  epi- 
thelial wall.  Drops  of  fat  occur  in  the  lymph  of  the 
mesentery  and  give  it  the  milky  character  which  long 
ago  fixed  the  name  of  lacteals  upon  these  vessels.  It  is 
known  that  most  of  the  fat  entering  the  circulation  after 
a  meal  traverses  the  thoracic  duct.  This  was  ascertained 
by  observing  cases  in  which  the  duct  had  been  severed  so 
that  the  lymph  stream  escaped  through  a  wound.  After 
a  meal  containing  a  known  amount  of  fat  about  two- 
thirds  of  it  could  be  recovered  in  the  collected  lymph. 

The  fat  which  is  generated  in  the  intestinal  wall  from 
the  cleavage-products  of  fat  previously  fed  and  digested 
is  not  a  mere  reproduction  of  the  original  fat.  It  has  a 
constant  composition  proper  to  the  species  and  inde- 
pendent of  variations  in  the  nature  of  the  fat  furnished 

303 


304  HUMAN    PHYSIOLOGY 

in  the  diet.  So  an  animal  eating  a  vegetable  oil  does 
not  store  the  same  oil  in  its  adipose  tissue,  but  accumu- 
lates there  a  fat  which  conforms  to  its  own  standard. 
The  only  exception  is  noted  when  a  fast  is  followed  by 
abundant  feeding;  at  such  a  time  there  may  be  some 
retention,  for  a  while,  of  a  distinctly  foreign  fat. 

Most  of  the  fat  in  the  body  is  in  what  we  have  called 
adipose  tissue.  When  we  spoke  of  peptic  digestion  we 
described  this  as  a  tissue  rich  in  fat  but  not  composed 
solely  of  that  material.  It  is  a  form  of  connective  tissue 
with  fibers  between  the  cells.  But  while,  as  a  rule,  the 
intercellular  substance  makes  up  the  bulk  of  any  con- 


FIG.    64. — Cells    in    adipose     tissue   distended   with   fat.      Note   the 
capillary,  fibers,  and  undeveloped  cells. 

nective  tissue  we  recognize  an  exception  in  this  case. 
The  fat  carried  is  intracellular,  that  is  to  say,  enclosed  in 
the  cells  instead  of  being  placed  between  them.  It  is  so 
abundant  that  the  cells  have  a  swollen  appearance,  their 
nuclei  are  pushed  to  the  surface,  and  their  true  proto- 
plasm is  a  mere  envelope  for  the  fat  drops  within. 

Adipose  tissue  is  found  even  in  lean  animals  to  an 
amount  not  usually  suspected.  A  considerable  mass  is 
normally  present  below  the  diaphragm  and  about  the 
kidneys.  The  white  marrow  in  the  hollow  shafts  of  the 
long  bones  is  essentially  adipose  tissue.  When  the 


METABOLISM  305 

pioneers  in  crossing  the  deserts  of  Nevada  killed  their 
famished  oxen  for  food  they  were  disappointed  to  find 
this  marrow  replaced  by  fluid;  the  animals  had  nearly 
exhausted  this  reserve  along  with  other  deposits.  Some 
adipose  tissue  is  usually  found  about  the  heart,  behind 
the  eyes,  and  in  the  mesentery. 

In  well-fattened  individuals  these  accumulations  are 
supplemented  by  others.  Chief  of  these  is  the  sub- 
cutaneous fat,  a  layer  between  the  skin  and  the  muscles. 
This  may  be  indefinitely  increased  and  its  prominence 
determines  our  judgment  as  to  whether  a  subject  is 
obese  or  not.  Corpulent  persons  have  much  adipose 
tissue  in  the  great  omentum,  making  this  appendage 
to  the  stomach  a  massive  affair  instead  of  the  filmy  apron 
which  it  is  ordinarily.  We  may  assume  that  the  average 
human  adult  has  at  least  4  or  5  pounds  of  fat  in  his 
system. 

The  ultimate  service  of  this  fat  is  to  be  oxidized  and  to 
contribute  its  energy  to  the  working  tissues,  particularly 
the  skeletal  muscles.  To  perform  this  function  it  must 
first  be  carried  from  the  seat  of  its  temporary  storage  to 
the  contractile  organs.  This  transfer  occurs  most 
actively  in  periods  of  fasting  when  the  organism  is 
feeding  upon  its  own  stores.  The  blood  of  a  starving 
animal  may  be  richer  in  fat  than  is  usual  during  mixed 
feeding  for  it  is  engaged  in  this  work  of  transportation. 

When  fat  is  fully  oxidized  the  only  end-products  are 
carbon  dioxid  and  water.  In  certain  pathologic  condi- 
tions, notably  in  severe  diabetes,  the  oxidation  of  fat  is 
incomplete  and  certain  less  simple  compounds  are  formed. 
These  are  of  an  acid  character  and  their  accumulation  in 
the  blood  and  elsewhere  may  be  responsible  for  a  pro- 
found poisoning.  This  is  appropriately  called  an 
acidosis.  Very  much  as  when  we  fail  to  burn  coal  cleanly 
and  completely  we  get  the  poisonous  carbon  monoxid, 
so  when  fat  is  not  effectually  oxidized  in  the  body  we 
have  generated  products  which  are  immeasurably  more 
injurious  than  the  normal  ones. 

20 


306  HUMAN    PHYSIOLOGY 

Carbohydrate  Metabolism. — We  have  seen  that  in 
the  course  of  digestion  starches  and  complex  sugars  are 
changed  to  sugars  of  the  simplest  order.  By  far  the 
most  abundant  of  these  simple  sugars  is  the  one  which  we 
call  dextrose.  It  is  identical  with  the  sugar  found  in 
grapes.  Our  account  of  carbohydrates  in  the  body 
resolves  itself  into  a  history  of  the  use  made  of  dextrose 
by  the  tissues.  Within  a  few  hours  after  a  meal  4  or  5 
ounces  of  this  sugar  may  enter  the  portal  circulation. 
This  sugar  will  be  offered  to  the  liver  cells  before  it  is 
presented  to  any  other  organ.  The  clue  is  an  important 
one  and  was  long  ago  followed  by  Bernard  with  fruitful 
results. 

The  liver  of  a  well-nourished  animal  contains  carbo- 
hydrate to  an  extent  that  other  organs  do  not  approach. 
The  particular  representative  of  the  carbohydrate  class 
is  more  like  a  starch  than  a  sugar :  it  is  of  high  molecular 
weight  and  limited  solubility.  It  is  found  within  the 
cells  of  the  liver  in  small  solid  clumps.  We  sometimes 
call  it  animal  starch  but  more  often  glycogen.  The  word 
means  " sugar  former"  and  the  reference  is  to  the  readi- 
ness with  which  glycogen  undergoes  digestion  and  is 
resolved  into  dextrose.  The  human  liver  may  contain 
as  much  as  6  ounces  of  glycogen ;  in  herbivorous  animals, 
such  as  the  rabbit,  the  proportion  may  be  higher  than  in 
man. 

Since  we  know  that  the  liver  stands  in  the  path  of  the 
incoming  carbohydrate,  and  since  we  find  this  organ  rich 
in  a  starch-like  substance,  it  is  natural  to  infer  that  some 
portion  of  the  absorbed  sugar  may  be  arrested  and 
retained  by  the  liver.  It  is  turned  into  glycogen  by  a 
change  which  is  a  condensation,  the  reverse  of  digestion. 
It  remains  subject  to  reconversion  to  dextrose,  in  which 
form  it  is  dealt  out  to  the  blood  in  times  of  fasting. 
There  is  a  definite  resemblance  between  the  liver,  so 
far  as  this  function  is  concerned,  and  a  tuber  like  a  potato. 
The  liver  and  the  potato  are  both  repositories  of  surplus 
carbohydrate  and  in  both  cases  the  material  enters  and 


METABOLISM  307 

leaves  in  the  form  of  sugar.  It  is  held  in  storage  in  the 
less  soluble  and  more  manageable  forms  of  glycogen 
and  starch  respectively.  While  glycogen  is  conspicuous 
in  the  liver  it  is  formed  and  retained  by  other  tissues  to 
some  extent.  The  skeletal  muscles,  in  particular,  con- 
tain it  in  a  small  percentage  but  a  large  total  quan- 
tity. They  constitute  a  mass  of  tissue  many  times  larger 
than  the  liver  and  they  are  supposed  to  contain  rather 
more  glycogen  than  the  great  gland  where  it  was  first 
found. 

Owing  to  the  tendency  of  the  liver  and  the  muscles  to 
make  glycogen  when  an  excess  of  sugar  is  brought  under 
their  influence  and  to  return  sugar  to  the  circulation 
when  there  is  no  influx  from  the  intestine  the  blood  is 
protected  against  alternate  surcharging  and  impoverish- 
ment. Its  sugar  content  does  not  rise  materially  unless 
a  great  deal  of  sugar  is  fed.  It  is  sometimes  possible, 
however,  to  exceed  the  capacity  of  the  tissues  for  storing 
glycogen  and  to  establish  for  a  short  time  a  condition  of 
hyperglycemia,  an  abnormally  high  percentage  of  sugar 
in  the  blood.  If  this  is  at  all  marked  some  sugar  will 
pass  into  the  urine ;  the  kidneys  will  abstract  it  from  the 
blood  if  the  liver  and  muscles  fail  to  do  so.  The  ap- 
pearance of  sugar  in  the  urine  in  consequence  of  eating 
largely  of  it  is  called  alimentary  glycosuria. 

If  the  carbohydrate  supply  of  the  body  is  for  some 
time  in  excess  of  the  current  consumption  the  formation 
of  glycogen  will  not  be  indefinitely  continued.  Instead, 
there  is  likely  to  be  a  transformation  of  some  of  the 
starch  and  sugar  consumed  into  body  fat.  The  maxi- 
mum storage  of  glycogen  is  probably  about  1  pound. 
When  this  limit  is  approached  conditions  are  favorable 
for  the  development  of  more  or  less  fat  from  carbohy- 
drate. This  possibility  was  once  denied  but  has  been 
proved  by  careful  experiments  as  well  as  by  practical 
experience.  We  have  every  reason  to  believe  that 
starches  and  sugars  are  responsible  for  much  of  the 
corpulence  which  is  such  a  common  and  serious  condition. 


308  HUMAN    PHYSIOLOGY 

It  was  once  held  that  body  fat  could  come  only  from 
fat  received  in  the  food.  This  view  was  absolutely 
disproved  by  a  single  argument  on  the  part  of  the  great 
chemist  Liebig.  He  called  attention  to  the  scant  supply 
of  fat  in  the  fodder  of  the  cow  as  compared  with  the 
large  daily  delivery  of  butter  fat  in  the  milk.  No  one 
could  maintain  the  old  belief  in  the  presence  of  these 
facts.  But  Liebig  merely  showed  that  fat  could  come 
from  something  other  than  itself,  and  it  was  not  at  once 
decided  whether  proteins  or  carbohydrates  were  to  be 
regarded  as  the  usual  source.  It  may  be  said  in  antici- 
pation of  what  is  to  come  later  that  proteins  are  possible 
fat-formers,  but  not  generally  so  prominent  as  the  carbo- 
hydrates. 

The  transformation  of  sugar  to  fat  being  a  well- 
established  and  frequent  occurrence,  it  becomes  natural 
to  inquire  whether  fat  can  be  changed  back  again  to 
sugar.  This  seems  easier  from  a  chemical  point  of  view 
than  the  former  reaction.  But  it  has  proved  difficult  to 
demonstrate  that  it  ever  happens.  There  is  at  present  a 
difference  of  opinion  in  regard  to  it  and  it  may  well  be 
considered  an  open  question. 

As  was  said  of  fat  so  it  may  be  said  of  carbohydrate 
that  its  essential  service  is  performed  when  it  is  oxidized 
with  liberation  of  energy.  There  is  a  further  cor- 
respondence between  the  two  in  that  the  oxidation  is 
mainly  effected  in  the  skeletal  muscles  and  that  the 
normal  end-products  are  carbon  dioxid  and  water. 

The  Pancreas. — The  oxidation  of  dextrose  stands  in  a 
curious  relationship  to  the  normal  activity  of  the 
pancreas.  We  have  dealt  with  that  organ  as  a  digestive 
gland  of  great  importance.  We  are  now  to  see  that  it 
has  another  function  even  more  fundamental.  About 
the  year  1889  certain  investigators  removed  the  pancreas 
from  dogs  to  observe  the  signs  of  deficiency  which  might 
make  their  appearance.  It  would  have  been  natural  to 
look  for  some  loss  of  digestive  capacity  and  a  lowered 
power  to  absorb  food.  Probably  these  results  ensued 


METABOLISM  309 

but  they  were  obscured  by  a  much  more  striking  conse- 
quence: the  loss  of  the  ability  to  oxidize  sugar. 

We  must  bear  in  mind  that  the  oxidation  of  sugar  does 
not  take  place  in  the  pancreas  and  yet  the  influence  of 
that  gland  is  one  of  its  necessary  conditions.  It  has  to 
be  assumed  that  some  agent  derived  from  the  pancreatic 
cells  enters  the  circulation  and  travels  to  the  tissues  far 
and  wide,  conferring  on  them  the  power  to  set  free  and 
utilize  the  energy  that  is  latent  in  the  sugar  molecules. 
The  agent  concerned  is  a  hormone  as  denned  in  Chapter 
XV.  It  is  not  precisely  an  enzyme  but  reminds  us  of 
that  type  of  substance  in  that  it  is  known  by  its  effects 
rather  than  as  an  isolated  body . 

Lack  of  the  pancreatic  hormone — with  consequent  lack 
of  the  capacity  to  make  use  of  dextrose — is  the  central 
condition  in  the  disease  diabetes.  This  is  popularly 
supposed  to  be  a  " kidney  trouble"  but  it  is  not.  When 
the  body  cannot  oxidize  sugar  the  continued  addition  of 
this  digestive  product  to  the  blood  leads  to  hypergly- 
cemia.  An  escape  of  sugar  in  the  urine  is  then  to  be 
expected,  for  normal  kidneys  always  let  it  pass  when  the 
concentration  on  the  blood  exceeds  a  certain  low  limit. 
Unlike  alimentary  glycosuria,  the  diabetic  state  means 
an  abundant  and  more  or  less  continuous  loss  of  sugar. 
In  fully  developed  cases  all  the  sugar  entering  the  blood 
is  transferred  to  the  urine  without  having  contributed  to 
the  activities  of  the  tissues.  * 

It  might  be  anticipated  that  in  diabetes  the  glycogen 
of  the  liver  and  muscles  would  be  maximal  in  amount. 
On  the  contrary  the  power  to  make  and  hold  this  deriva- 
tive of  sugar  seems  to  be  lost  along  with  the  ability  to 
oxidize.  The  pancreatic  hormone  appears  to  confer 
both  the  power  to  oxidize  sugar  and  the  power  to  convert 
it  into  glycogen.  When  diabetes  reaches  its  full  intensity 
and  no  dextrose  can  be  broken  down  there  follows,  as 
already  noted,  a  faulty  fat  metabolism  and  acidosis  of  the 
gravest  kind. 


310  HUMAN    PHYSIOLOGY 

Alcohol. — Alcohol  when  taken  is  absorbed  rapidly 
and  rather  quickly  oxidized.  It  yields  up  heat  and  gives 
rise  to  carbon  dioxid  and  water.  It  is  not  known  to  be 
transformed  in  fat  or  glycogen  so  there  is  no  apparent 
provision  for  its  storage.  The  familiar  fattening  effect 
of  alcoholic  drinks  is  indirect.  It  can  be  referred  to  two 
circumstances:  in  the  first  place  moderate  drinking 
creates  a  keen  appetite  and  so  favors  overeating; 
second,  when  alcohol  is  oxidized  in  the  body  there  is  less 
call  for  the  oxidation  of  fat  or  carbohydrate  to  meet  the 
current  need.  Fat  is  thus  " spared"  to  accumulate,  or 
carbohydrate  to  be  transformed  into  it. 

Nitrogenous  Metabolism. — An  equivalent  for  this 
title  would  be  The  History  of  the  Amino-acids.  No  one 
can  pursue  this  subject  far  without  the  fullest  command 
of  the  facts  of  biochemistry.  Our  treatment  must  be 
condensed  and  admittedly  superficial.  We  have  said 
that  the  amino-acids  are  the  structural  units,  the  "  build- 
ing stones,"  of  protein.  What  we  call  a  single  protein, 
for  instance,  the  albumin  of  white  of  egg,  yields  a  con- 
siderable number  of  amino-acids  when  it  is  thoroughly 
digested.  The  total  number  known  is  about  twenty. 

These  diverse  substances  pass  from  the  intestine  into 
the  portal  blood-stream.  It  was  once  held  that  they 
were  immediately  combined  to  form  proteins  of  the 
types  native  in  the  plasma  This  is  no  longer  believed; 
while  there  must  be  some  synthesis  of  new  proteins 
from  the  amino-acids  it  seems  to  be  quite  limited.  Of 
course  it  must  be  greater  during  the  period  of  growth  than 
it  is  in  a  body  which  is  no  longer  increasing  in  size.  It 
never  ceases  entirely  for  the  proteins  of  the  muscles  and 
glands  are  subject  to  a  gradual  disintegration  so  long  as 
life  lasts  and  the  losses  need  to  be  made  good. 

According  to  one  conception  the  proteins  of  the  plasma 
are  offered  to  the  tissues  and  appropriated  by  them  as 
may  be  required.  To  turn  the  proteins  of  the  blood  into 
those  of  muscles  it  has  been  supposed  that  a  local  diges- 
tion is  carried  on  and  the  amino-acids  combined  accord- 


METABOLISM  13 

^ 

ing  to  a  new  pattern.     A  more  recent  view  L 
simpler.     This  is  to  the  effect  that  it  is  the  L 
acids  rather  than  the  proteins  of  the  plasma  on  \ 
various  tissues  depend  for  their  renewal.     We          ieft 
somewhat  in  doubt  as  to  the  real  value  of  the  blood 
proteins. 

During  starvation  it  is  known  that  some  organs  are 
sustained  while  others  are  suffered  to  waste.  Thus  the 
heart  and  the  brain  are  preserved  almost  intact  while  the 
spleen  and  the  liver  lose  largely  in  weight.  At  such 
times  we  may  picture  the  proteins  of  the  less  essential 
organs  becoming  resolved  into  amino-acids  which  can  be 
incorporated  into  those  which  must  be  protected.  A 
striking  case  is  that  of  the  female  salmon  when  the  time 
for  spawning  is  approaching.  The  muscles  steadily 
atrophy  and  there  is  no  doubt  that  their  substance  is 
made  to  contribute  to  the  growing  mass  of  roe. 

It  used  to  be  held  that  one  protein  must  be  equivalent 
to  another.  The  impression  was  a  natural  one  so  long  as 
attention  was  directed  only  to  the  percentage  composition 
of  proteins  from  different  sources.  Their  content  of 
oxygen,  nitrogen,  carbon,  etc.,  varies  but  little.  When 
the  significance  of  the  " building  stones"  came  to  be 
appreciated  it  became  clear  that  certain  proteins  may 
be  much  more  serviceable  than  others  for  the  general 
work  of  nutrition.  If  they  do  not  furnish  all  the  con- 
structive units  called  for  they  may  be  hopelessly  inade- 
quate. One  example  of  an  inadequate  or  defective 
protein  has  long  been  known.  This  is  gelatin  from  the 
connective  tissue  of  meat  and  from  bone. 

Gelatin  was  found  long  ago  to  analyze  like  protein  in 
general.  It  seemed  to  carry  the  standard  quantity  of 
nitrogen  and  the  other  elements.  Yet  it  never  could 
replace  other  proteins  in  the  diet  of  animals  or  men 
without  initiating  a  decline  of  weight  and  condition  that 
would  continue  to  the  end  of  the  trial.  We  have  learned 
now  that  the  difficulty  with  gelatin  is  the  absence  of 
one  or  two  amino-acids  from  the  list  of  its  cleavage- 


310  HUMAN    PHYSIOLOGY 

products.  It  fails  to  furnish  certain  "  building  stones" 
which  are  indispensable.  There  are  a  number  of 
vegetable  proteins  which  resemble  gelatin  in  their 
failure  to  supply  important  amino-acids.  Their  exist- 
ence was  not  evident  until  special  studies  demonstrated 
it,  because  they  were  found  in  close  association  with  other 
proteins  of  a  superior  sort  from  which  they  had  to  be 
separated  before  they  could  be  tested  as  individuals. 

As  there  are  proteins  which  cannot  satisfy  all  the 
requirements  of  animal  life,  so  there  are  others  which  can 
do  so,  but  only  when  consumed  in  what  seems  an  ex- 
travagant manner.  The  trouble  with  these  proteins  is 
that  while  they  give  all  the  needful  amino-acids,  they 
yield  them  in  proportions  not  corresponding  to  the 
demand.  If  the  body  needs  a  good  deal  of  an  amino-acid 
which  a  certain  protein  furnishes  but  scantily  the  total 
quantity  of  the  protein  must  be  increased  until  the 
particular  want  is  met.  When  this  condition  is  realized 
there  will  be  an  excessive  offering  of  other  amino-acids 
for  which  the  organism  has  no  distinct  use. 

In  the  light  of  what  has  been  said  it  should  not  be 
suprising  to  learn  that  some  proteins  are  much  superior 
to  others  when  the  judgment  is  based  on  the  minimum 
amount  serving  to  keep  a  subject  from  loss  of  tissue. 
Generally  speaking,  the  proteins  of  meat  excel  those  of 
vegetable  origin  in  their  ability  to  maintain  the  nutri- 
tional balance  with  economy.  But  the  proteins  of  rice, 
milk,  and  potato  are  nearly  as  good.  Larger  quantities 
of  the  proteins  of  wheat  and  beans  must  be  fed  to  secure 
the  same  result,  while  the  proteins  of  Indian  corn  are 
among  the  most  wasteful  which  have  come  under 
observation. 

It  is  important  that  the  implications  of  what  has  been 
stated  shall  not  be  misunderstood.  The  facts  were 
ascertained  through  experiments  in  course  of  which 
volunteers  were  restricted  to  one  type  of  protein  at  a 
time.  We  do  not  commonly  limit  ourselves  in  any  such 
a  way.  Because  it  takes  a  great  deal  of  the  proteins 


METABOLISM  313 

from  corn  to  meet  the  needs  of  the  system  we  are  not  to 
conclude  that  corn  is  a  poor  food.  It  may  be  one  of  the 
best  possible  to  mix  with  some  other  which  will  give  quite 
different  percentages  of  the  amino-acids.  Moreover,  if 
we  admit  financial  considerations  it  will  be  necessary  to 
bear  in  mind  that  much  more  of  the  cereal  proteins  can 
be  had  for  a  dollar  than  of  those  in  meat,  milk,  and  eggs. 

Urea. — We  usually  eat  much  more  protein  than  is 
absolutely  necessary.  Even  if  we  do  not,  there  are  bound 
to  be  amino-acids  in  the  circulation  for  which  there  is 
no  present  demand.  These  are  not  wasted  altogether. 
The  nitrogen  which  they  contain  is  withdrawn  from  their 
molecules,  not  as  an  element  but  in  the  compound  urea. 
This  is  a  soluble,  non-irritating  compound  which  is 
excellently  adapted  to  be  the  chief  nitrogeneous  excretory 
product.  Much  of  it  is  formed  in  the  liver,  but  much  is 
also  manufactured  in  the  tissues  at  large.  It  circulates 
until  removed  from  the  blood  by  the  kidneys. 

After  the  formation  of  urea  has  been  accomplished  the 
residue  of  the  amino-acid  molecules  seems  to  have  a 
history  identical  with  that  of  carbohydrate  in  the 
metabolism.  In  fact  a  considerable  part  of  the  proteins 
we  eat  seems  to  exist  later  as  sugar.  Hence  there  is  a 
possibility  of  glycogen  formation  from  protein  and,  as 
previously  suggested,  possible  transformation  into  fat 
of  some  of  the  protein  sugar.  In  diabetes  the  sugar 
formed  from  protein  comes  to  light  and  makes  its  ap- 
pearance even  in  fasting  when  the  proteins  of  the  body 
are  being  consumed.  It  will  be  realized  that  a  diabetic 
suffers  not  only  from  the  loss  of  the  power  to  utilize 
carbohydrates,  but  is  deprived  to  a  great  extent  of  the 
support  of  protein.  When,  at  the  last,  he  can  no  longer 
oxidize  fat  successfully  the  problem  of  his  nutrition  is 
hopeless. 

To  work  over  the  amino-acids  in  such  a  way  as  to  sepa- 
rate urea  from  them  and  then  to  turn  the  remainder  into 
dextrose  may  seem  a  roundabout  and  costly  mode  of 
getting  sugar.  This  is  a  fair  argument  against  the  over- 


314  HUMAN    PHYSIOLOGY 

consumption  of  proteins,  but,  on  the  other  hand,  since 
there  are  bound  to  be  amino-acids  not  needed  for  syn- 
thetic uses  it  is  clearly  better  to  make  fuel  of  them  than  to 
reject  them  wholly.  If  a  man  pulls  down  an  old  house 
and  builds  a  new  one  out  of  the  timbers  he  is  pretty  sure 
to  have  many  misfit  pieces.  It  is  the  part  of  common 
sense  to  throw  these  into  the  cellar  to  be  burned  when 
desired.  The  body  is  operated  according  to  the  very 
same  principle. 

We  have  distinguished  between  adequate  and  inade- 
quate proteins.  Recent  investigations  have  shown  that 
a  three-fold  classification  is  more  precise.  Some  proteins 
are  obviously  insufficient,  gelatin  being  an  example. 
Some  answer  for  the  maintenance  of  weight  but  not  for 
growth.  The  proteins  of  the  highest  order  are  those 
which  will  promote  the  growth  of  young  animals. 


CHAPTER  XXIII 
EXCRETION 

The  waste  of  the  body  consists  mainly  of  highly  oxi- 
dized products.  If  the  economic  ideal  were  attained 
these  products  should  represent  no  potential  energy. 
The  major  ones  do  not,  but  some  of  the  minor  ones  are 
susceptible  of  further  oxidation.  Carbon  dioxid  is  the 
foremost  product  of  the  metabolism  and  its  removal 
from  the  system  has  been  discussed  in  the  chapters  on 
Respiration. 

Water. — Water  occupies  a  peculiar  position  since  it 
may  be  claimed  that  it  is  both  a  food  and  a  waste.  All 
the  water  that  is  taken  into  the  body — neglecting  that 
portion  which  may  be  added  to  the  tissues  during  growth — 
will  be  discharged  again.  The  total  output  will  normally 
be  larger  than  the  intake,  for  the  water  that  passes  into 
and  out  from  the  body  has  united  with  it,  at  the  seat  of 
respiration,  a  moderate  amount  of  water  formed  by 
oxidation.  This  smaller  quantity  has,  of  course,  no 
quality  to  distinguish  it  from  the  greater  volume  of 
water  in  which  it  is  merged.  At  times  when  the  body  is 
gaining  water  there  may  be  no  excess  of  outgo  over 
income.  Water,  while  in  part  a  waste-product  itself, 
is  most  useful  as  a  bearer  of  other  waste  in  solution. 
It  figures  thus  in  the  urine  and  to  some  extent  in  the 
perspiration. 

The  chief  ways  by  which  excretion  can  go  on  are  four : 
by  the  breathing,  the  urine,  the  feces,  and  the  sweat. 
The  order  can  be  defended  on  the  ground  that  obstruction 
of  the  breathing  is  more  immediately  fatal  than  suppres- 
sion of  the  urine,  while  this  in  its  turn  is  more  serious 
than  the  failure  of  intestinal  elimination.  Contrary 

315 


316 


HUMAN    PHYSIOLOGY 


to  popular  belief,  the  secretions  of  the  skin  have  the 
slightest  share  in  the  total  work  of  excretion.  The  great 
value  of  the  sweat  glands  is  not  at  all  in  connection  with 
the  removal  of  waste  but  in  the  dissipation  of  heat. 


•  FIG.  65. — The  kidneys  and  the  urinary  bladder.  The  two  kidneys 
are  shown  within  an  outline  which  suggests  the  body  cavity.  Their 
advantageous  connections  with  the  chief  artery  and  vein  of  the  system 
are  indicated.  Below  is  the  bladder  reached  by  the  two  ureters.  These 
vessels  enter  the  bladder  low  down  and  behind — not  at  the  level  where 
they  disappear  from  the  figure. 

The  expired  air  carries  out  from  the  body  nearly  all  the 
carbon  dioxid  and  a  rather  large  volume  of  water,  some- 


EXCRETION  317 

times  as  much  as  a  pint  in  twenty-four  hours.  The 
kidneys  often  discharge  as  much  as  3  pints  of  water 
in  the  same  length  of  time.  Dissolved  in  it  is  the  larger 
part  of  the  mineral  matter  requiring  to  be  eliminated. 
More  important  than  this  is  the  presence  in  the  urine  of 
compounds  of  nitrogen,  sulphur,  and  phosphorus,  the 
distinctive  end-products  of  protein  decomposition.  The 
chief  of  these  bodies  is  the  compound  urea  which  was 
mentioned  in  the  last  chapter.  Experience  indicates 
that  students  must  be  cautioned  against  using  urea  and 
urine  as  synonyms.  Urea  is  the  leading  substance  in 
solution  in  urine,  but  the  two  words  are  not  inter- 
changeable. 

The  waste  from  the  intestine'  is  not  easily  defined.  It 
is  of  a  miscellaneous  character;  the  bile  pigments  in  a 
modified  form  are  examples  of  excretions  passing  from 
the  body  by  this  route.  The  water  loss  from  the 
alimentary  canal  is  normally  small.  As  to  the  skin,  the 
perspiration  is  approximately  a  mineral  secretion  con- 
taining little  dissolved  matter  besides  common  salt. 
When  it  is  profuse  it  may  carry  in  very  small  amounts 
organic  waste-products  like  those  of  the  urine.  These 
are  somewhat  increased  when  there  is  kidney  disease,  but 
the  skin  can  by  no  means  compensate  for  the  loss  of  the 
renal  function. 

The  Work  of  the  Kidneys. — We  have  now  to  enlarge 
upon  the  work  of  the  kidneys.  These  are  paired  organs 
placed  to  the  right  and  left  of  the  vertebral  column  just 
below  the  diaphragm.  The  aorta  and  the  inferior  vena 
cava  pass  between  them.  A  large  but  short  artery  leads 
from  the  aorta  to  either  kidney;  a  corresponding  vein 
connects  each  kidney  with  the  vena  cava.  Thus  the 
kidneys  are  in  a  position  which  favors  a  copious  flow  of 
blood  through  their  vessels.  A  short  cut  or  shunt  for  the 
blood  is  opened  through  them.  Their  actual  blood-supply 
in  proportion  to  their  mass  is  exceptionally  large.  It  is 
said  to  be  exceeded  in  only  one  organ,  the  thyroid  gland. 

The  microscopic  details  of  the  kidney  are  of  such  com- 


318  HUMAN    PHYSIOLOGY 

plexity  that  we  shall  not  undertake  to  present  them. 
The  secreting  units  are  long  and  tortuous  tubules  originat- 
ing near  the  surface  of  the  organ  and  conducting  the 
urine  toward  a  cavity  in  the  concave  border.  A  funnel- 
shaped  appendage  receives  the  urine  and  it  passes  on  into 
the  ureter,  a  slender  and  delicate  yet  definitely  contractile 
vessel.  The  muscular  elements  of  the  ureter  are  of 
the  smooth  variety  and  they  execute  a  true  peristalsis. 
The  travelling  contractions  propel  the  urine  in  small 
quantities  to  the  bladder. 

This  is  a  contractile  sac  placed  in  the  pelvis  in  front  of 
the  rectum.  The  two  ureters  enter  it  low  down  and 
behind.  Their  openings  are  not  far  from  that  through 
which  the  urine  escapes  to  the  exterior.  This  passage  is 
the  urethra.  The  three  openings  are  at  the  angles  of  a 
small  triangle  which  is  not  much  disturbed  by  the 
alternate  enlargement  and  contraction  of  the  bladder. 
We  have  used  this  organ  before  (Chapter  IV)  to  illustrate 
what  is  meant  by  change  of  tone.  When  it  has  just  been 
emptied  it  is  quite  inconspicuous  and  its  upper  surface  is 
practically  in  contact  with  its  base.  When  it  is  full  it  is 
rounded  up  and  its  walls  are  thin  as  though  stretched. 
Yet,  as  we  have  insisted,  we  must  not  think  that  they  are 
really  stretched  unless  the  condition  is  extreme;  they 
have  relaxed  or  lost  tone  and  are  not  necessarily  reacting 
with  much  pressure  upon  the  liquid  inside. 

The  bladder  is  often  involved  in  reflexes.  It  is  apt  to 
contract  when  the  hands  are  dipped  in  water.  The  ure- 
thra is  controlled  by  muscle,  both  striped  and  smooth, 
acting  on  the  principle  of  a  sphincter.  When  this 
sphincter  is  voluntarily  or  otherwise  inhibited,  the  urine 
enters  the  passage  and  seems  to  evoke  a  reflex  contrac- 
tion of  the  bladder  which  rapidly  completes  the  dis- 
charge. Extra  pressure  may  be  thrown  upon  the  bladder 
by  contracting  the  abdominal  muscles. 

The  Urine. — Some  characteristics  of  the  urine  may  now 
be  pointed  out.  Its  color  is  due  to  the  carrying  over  to 
the  kidneys  of  substances  having  their  origin  in  the  liver 


EXCRETION  319 

and  related  to  the  pigments  of  the  bile.  The  color  is 
deepened  in  jaundice  when  the  escape  of  the  bile  pig- 
ments by  the  normal  channel  is  interfered  with.  It  more 
commonly  varies  with  the  degree  of  dilution.  After 
lively  perspiration  without  water  drinking  the  urine  is 
apt  to  be  high-colored,  its  dissolved  solids  being  held  in 
a  small  volume. 

The  urine  of  man  when  fresh  is  acid  by  ordinary 
standards  of  measurement.  It  becomes  alkaline  on 
standing  because  of  the  bacterial  fermentation  of  urea 
with  formation  of  ammonium  carbonate.  When  this 
change  is  advanced  an  ammoniacal  odor  develops  and 
a  cloudy  deposit  may  appear.  The  sediment  noticed 
under  these  conditions  is  nothing  abnormal.  The  urine 
of  herbivorous  animals  is  alkaline  even  when  fresh  unless 
the  animals  are  deprived  of  food.  It  then  becomes  acid 
and  the  metabolism  will  naturally  have  changed  to  a 
carnivorous  type,  the  animals  living  at  the  expense  of 
their  own  tissues. 

With  an  average  diet  we  may  expect  that  about  seven- 
eighths  of  the  nitrogen  represented  by  the  compounds  of 
the  urine  will  be  in  the  form  of  urea.  The  remainder  is 
divided  among  several  waste-products  all  more  complex 
than  urea  and  not  profitably  to  be  discussed  without  the 
assumption  of  a  knowledge  of  organic  chemistry.  One  of 
these  minor  bodies  is  uric  acid,  a  substance  distinguished 
by  its  scant  solubility  and  consequent  tendency  to  be 
retained  in  the  tissues.  Some  is  inevitably  formed  in 
the  daily  metabolism,  but  the  amount  can  be  kept  down 
when  desirable  by  temperance  in  protein  feeding  and 
especially  in  the  consumption  of  meat. 

All  proteins  contain  sulphur  and  some  contain  phos- 
phorus. Accordingly,  when  they  disintegrate  in  the 
body  these  elements  have  to  be  removed.  Like  the  ni- 
trogen they  do  not  go  free  but  in  combinations,  the 
sulphur  in  several  forms  but  chiefly  as  sulphates,  the 
phosphorus  almost  wholly  as  phosphates.  These  salts 
occur  in  urine  together  with  a  considerable  amount  of 


320  HUMAN    PHYSIOLOGY 

sodium  chlorid  (common  salt),  and  it  is  well  to  emphasize 
the  essential  difference  between  the  history  of  the  phos- 
phates and  sulphates  on  the  one  hand  and  the  chlorids 
on  the  other.  The  former  are  oxidized  products  of 
proteins;  the  chlorids  are  like  most  of  the  water  of  the 
excreta — merely  matter  which  was  previously  received 
in  the  same  state.  When  no  chlorids  are  fed  the  elimina- 
tion is  soon  reduced  to  a  very  low  level. 

The  quantity  of  the  urine  is  influenced  by  many  factors, 
but  most  radically  by  the  amount  of  water  taken  and  the 
varying  activity  of  the  sweat  glands.  A  hot  day  is 
likely  to  mean  a  contracted  urine,  but  some  people  drink 
enough  extra  water  in  warm  weather  to  provide  for  a 
good  volume  in  spite  of  the  large  quantity  passing  out 
through  the  skin.  A  sudden  cooling  of  the  body  with  a 
check  on  the  perspiration  can  be  depended  on  to  increase 
kidney  activity.  This  is  most  striking  when  one  leaves 
the  hot  land  and  goes  to  sea  on  a  summer  day. 

We  must  not  assume  to  judge  of  the  actual  work  done 
by  the  kidneys  by  observing  how  much  urine  they  secrete. 
It  is  altogether  probable  that  these  organs  are  most 
severely  taxed  when 'they  have  to  remove  from  the  blood 
a  maximum  of  dissolved  solids  in  a  minimum  of  water. 
In  other  words,  concentration  rather  than  volume  must 
be  our  criterion.  Average  urine  is  two  or  three  times  as 
concentrated  as  the  blood  from  which  it  is  derived. 
Students  of  physical  chemistry  tell  us  that  the  separation 
of  two  liquids  of  unequal  concentration  requires  the 
application  of  energy  in  perfectly  definite  and  large 
amounts.  The  implication  is  that  we  shall  favor  the 
kidneys  by  diluting  the  urine  so  that  it  shall  not  so 
markedly  surpass  the  concentration  of  the  blood. 

The  precepts  of  renal  hygiene  are  few  and  plain.  Drink 
plenty  of  water.  Do  not  eat  protein  foods  to  excess. 
Do  not  eat  a  great  deal  of  salt.  It  is  not  so  easy  to 
apply  these  directions  for  there  is  no  agreement  as  to  the 
protein  standard  or  how  much  salt  is  "a  great  deal." 
The  protein  question  will  be  given  further  attention. 


EXCRETION 


321 


We  have  learned  that  some  glands  are  very  clearly 
under  the  command  of  the  central  nervous  system  while 
others  are  influenced  more  particularly  by  the  changing 
volume  of  the  blood-stream  which  penetrates  their 
vessels.  The  kidney  is  a  gland  of  the  latter  type. 
Its  activity  is  usually  in  proportion  to  its  blood-supply. 
A  general  rise  of  arterial  pressure  will  drive  the  blood 
more  rapidly  than  before 
through  the  renal  capillaries 
and  the  response  is  likely  to 
be  prompt  and  considerable. 
The  explanation  of  the  in- 
fluence of  cold  upon  the 
kidneys  can  probably  be  ex- 
plained on  this  principle. 
There  is  first  a  constriction  of 
the  surface  vessels.  This  may 
lead  to  a  rise  of  pressure  in  the 
aorta  and  so  to  an  accelerated 
kidney  circulation.  It  is  also 
possible  that  when  the  surface 
vessels  are  narrowed  those  of 
the  deeper  parts,  including  the 
kidneys,  are  dilated  through 
nervous  influence.  In  this 
case  there  might  be  no  rise  of 
aortic  pressure,  but  the  new 
distribution  of  blood  would 
secure  for  the  kidneys  a  larger  share  than  before. 

It  may  be  said,  in  passing,  that  the  sweat  glands  are 
under  direct  nervous  control.  While  there  is  usually  a 
correspondence  between  the  degree  of  their  activity  and 
the  blood-supply  of  the  skin,  this  is  not  always  true. 
We  know  that  there  may  be  cold  sweating  when  the  skin 
is  pale  and  manifestly  receiving  but  little  blood.  There 
is  other  evidence  to  the  same  effect :  that  the  glands  may 
produce  much  perspiration  with  a  restricted  allowance 


FIG.  66. — A  sweat  gland 
wrapped  about  with  capillaries 
is  suggested.  It  is  shown  as 
though  there  were  no  other 
structures  below  the  skin  up  to 
which  the  sweat  duct  leads. 


322  HUMAN    PHYSIOLOGY 

of  blood  and,  again,  may  secrete  less  than  usual  when  the 
skin  is  flushed  and  burning. 

The  Urine  and  the  Metabolism. — Our  statements  thus 
far  have  been  qualitative  rather  than  quantitative.  We 
must  now  begin  to  make  some  use  of  figures  and  it  will 
be  best  to  adopt  metric  standards.  The  gram  will  be 
our  common  unit  of  weight  and  if  it  is  an  unfamiliar  one, 
the  reader  has  only  to  bear  in  mind  that  about  28  grams 
make  an  ounce.  It  is  desirable  at  this  time  to  show  what 
can  be  learned  about  the  course  of  events  in  the  body  by 
analyzing  the  urine. 

The  datum  most  often  sought  is  the  quantity  of  nitro- 
gen contained  in  the  day's  urine.  We  use  this  figure  to 
estimate  the  amount  of  protein  which  has  been  decom- 
posed in  twenty-four  hours.  This  involves  at  least  two 
assumptions:  first,  that  all  the  nitrogen  excreted  by  the 
body  is  to  be  found  in  the  urine  and,  second,  that  all  the 
decomposition  products  of  protein  reach  the  exterior  quite 
promptly.  The  first  assumption  is  not  strictly  justified 
for  there  is  an  appreciable  loss  of  nitrogen  in  the  feces 
and  a  slight  one  through  the  skin.  The  second  suppo- 
sition, too,  may  not  be  wholly  allowable,  but  so  long  as 
we  are  content  with  approximations  we  may  regard  the 
nitrogen  of  the  urine  as  the  index  of  protein  metabolism. 

Suppose  that  the  urine  for  the  day  contains  12  grams 
of  nitrogen.  Nitrogen  constitutes  about  16  per  cent,  of 
an  average  protein.  To  see  how  much  protein  must  have 
been  destroyed  to  yield  12  grams  of  nitrogen  we  divide 
12  by  16  and  multiply  by  100  or,  what  is  the  same  thing, 
we  multiply  12  by  6.25.  Our  answer  is  75  grams.  The 
subject  under  observation  has  therefore  lost  at  least  75 
grams  of  protein  during  the  day  of  the  trial.  He  may 
have  lost  5  or  10  grams  more  than  this  owing  to  the  escape 
of  minor  quantities  of  nitrogen  in  the  feces  and  the  sweat. 
These  are  figures  of  average  magnitude  for  American 
students. 

Nitrogen  Equilibrium. — We  cannot  analyze  the  food 
which  a  man  eats,  but  we  can  analyze  some  more  that 


EXCRETION  323 

closely  resembles  it.  If  such  a  check  as  this  has  been 
kept  upon  the  diet  of  our  imaginary  subject  it  will 
probably  be  found  that  the  nitrogen  of  the  income  has 
been  very  nearly  the  same  as  the  nitrogen  of  the  outgo. 
Furthermore,  if  we  were  to  continue  the  experiment  for 
several  days  and  take  pains  to  vary  the  nitrogen  of 
the  food  supply  as  widely  as  possible  we  should  find  that 
the  excretion  would  adjust  itself  to  the  changes  in  the 
diet  with  but  little  lag. 

At  one  period  of  our  investigation  we  might  encourage 
the  eating  of  meat;  eggs,  and  legumes  to  insure  a  high- 
protein  ration.  We  should  find  that  if  30  grams  of 
nitrogen  could  be  tolerated  nearly  as  much  would  re- 
appear in  the  urine.  The  other  extreme  would  be 
reached  with  nearly  non-nitrogenous  diet.  This  has 
been  closely  approached  by  feeding  nothing  but  corn- 
starch  pudding  with  cream  and  sugar.  In  this  case  we 
should  not  have  the  customary  equilibrium  for  there 
would  still  be  a  definite  output  of  nitrogen  in  the  ab- 
sence of  income.  But  within  very  wide  limits  the 
body  excretes  just  about  as  much  nitrogen  as  it  receives. 

The  facts  ought  to  be  intelligible  in  the  light  of  what 
has  been  said  about  the  way  in  which  the  organism  deals 
with  proteins.  It  has  been  pointed  out  that  the  require- 
ment of  amino-acids  for  synthetic  service  is  a  very 
moderate  one.  No  matter  how  greatly  we  exceed  it 
we  find  that  the  system  responds  continually  in  the 
same  fashion:  it  sets  aside  for  excretion  all  the  surplus 
nitrogen.  Hence  the  balance  is  generally  struck  be- 
tween the  food  and  the  excreta  unless  the  income  is 
distinctly  deficient.  Retention  of  nitrogen  occurs  dur- 
ing growth  and  in  the  related  conditions  of  recov- 
ery from  fasting,  convalescence  from  illness,  and  in 
pregnancy. 

As  we  can  calculate  the  protein  decomposed  from 
the  nitrogen  excreted,  we  can  equally  well  calculate 
the  storage  of  protein  in  the  body  of  growing  animals 
from  the  quantity  of  nitrogen  retained.  If,  in  the  course 


324  HUMAN    PHYSIOLOGY 

of  a  week  or  two,  an  animal  has  received  20  grams  more 
nitrogen  than  it  has  given  back  to  its  environment  the 
inference  is  that  it  has  synthesized  from  the  products 
of  digestion  (20  X  6.25)  or  125  grams  of  new  protein. 
If  this  were  all  represented  by  muscle  the  total  would 
be  about  625  grams,  for  muscle  is  about  75  per  cent, 
water  and  contains  perhaps  5  per  cent,  of  other  non- 
protein  matter,  in  other  words,  one-fifth  of  it  is  protein. 
The  increase  in  weight  of  a  growing  organism  will  ac- 
cordingly be  several  times  as  great  as  the  protein  storage. 
It  must  add  water  and  mineral  salts  to  make  new  tissue 
and  it  is  likely  to  add  some  fat. 

It  will  be  apparent  also  that  when  nitrogen  is  lost 
from  the  body,  as  in  fasting,  the  weight  must  fall  by 
an  amount  many  times  greater  than  that  of  the  nitrogen 
excreted.  The  reasoning  is  parallel  to  that  employed 
above:  a  loss  of  1  gram  of  nitrogen  means  a  loss  of 
6.25  grams  of  protein,  and  this  in  its  turn  stands  for  the 
destruction  of  perhaps  30  grams  of  average  tissue. 
A  fasting  animal  will  therefore  excrete  an  amount  of 
water  which  cannot  be  accounted  for  on  the  basis  of 
income  or  oxidation;  it  is  water  set  free  by  the  dissolu- 
tion of  tissue.  This  is  a  fact  often  overlooked. 

The  composition  of  the  urine  depends  more  upon  the 
protein  intake  than  upon  any  other  factor.  -It  will  be 
well  to  emphasize  just  here  a  negative  statement, 
namely,  that  the  proportion  and  quantity  of  the  urinary 
constituents  are  but  slightly  influenced  by  muscular 
activity.  This  is  the  same  as  saying  that  protein  de- 
composition is  not  materially  increased  by  exercise. 
The  observation  is  an  interesting  one  because  muscle 
is  so  largely  made  of  protein  that  early  writers  naturally 
assumed  that  this  kind  of  material  must  be  sacrificed  in 
the  act  of  contraction.  It  has  become  clear  that  in  our 
consideration  of  muscle  we  must  distinguish  between 
the  machine  and  the  fuel.  The  machine  is  constructed 
chiefly  of  proteins  (with  water  and  salts),  but  the  pre- 
ferred fuel  is  sugar,  with  fat  also  available. 


EXCRETION  325 

Normal  urine,  as  we  have  seen,  affords  a  basis  for 
the  estimation  of  nitrogenous  metabolism.  Abnormal- 
ities in  the  working  of  the  body  are  often  registered 
by  this  secretion.  The  significance  of  sugar  has  been 
dwelt  upon  but  may  here  be  restated  briefly.  A  transient 
occurrence  may  result  from  eating  much  sugar  and  is 
not  a  sign  of  disease.  If  sugar  is  often  present  and  in 
some  quantity  the  suggestion  is  that  the  body  lacks 
the  full  power  to  oxidize  this  food.  Under  such  circum- 
stances the  reduction  of  carbohydrate  in  the  diet  for 
a  while  is  often  beneficial.  In  fully  developed  diabetes 
little  or  no  sugar  is  oxidized  and  glycosuria  is  continuous. 
Not  only  the  dextrose  formed  from  the  carbohydrates 
of  the  food,  but  much  that  is  attributed  to  protein 
sources  then  comes  to  light. 

Emotional  Glycosuria. — It  has  lately  been  shown  that 
sugar  may  be  found  in  the  urine  of  entirely  healthy 
subjects  after  an  emotional  strain.  This  has  been 
noted,  for  example,  in  students  who  have  taken  hard 
examinations  and  in  athletes  who  have  either  played 
in  crucial  games  or  waited  to  be  called  on  as  substitutes. 
The  relation  between  the  nervous  system  and  the 
observed  result  is  somewhat  indirect  and  may  be  re- 
ferred to  later  on.  For  the  present  purpose  it  may  be 
said  that  during  excitement  influences  are  brought  to 
bear  upon  the  liver  cells  which  lead  to  the  transformation 
of  much  glycogen  into  sugar.  This  newly  formed  sugar 
enters  the  circulation  creating  a  condition  of  hyper- 
glycemia  which,  as  we  have  already  learned,  will  cause 
some  leakage  of  sugar  through  the  kidney  substance. 

In  the  urine  of  the  diabetic  there  will  be  found,  in 
advanced  cases,  not  only  sugar  but  an  overflow  of"  the 
acid  products  of  an  imperfect  fat  metabolism.  These 
poisons  are  collectively  known  as  the  acetone  bodies. 
Protein  itself  (albumin)  frequently  makes  its  appearance 
in  the  urine  and  is  often,  though  not  by  any  means 
always,  associated  with  disease  of  the  kidneys.  Let 
it  be  repeated  that  diabetes  is  not  a  kidney  disorder 


CHAPTER  XXIV 
INCOME  AND  OUTGO 

The  main  facts  about  the  changes  intervening  be- 
tween the  absorption  of  food  and  the  discharge  of  the 
corresponding  waste-material  must  now  be  clear.  If 
we  disregard  the  processes  of  growth  and  think  of  our 
food  as  fuel  we  can  say  that  it  is  oxidized,  either  promptly 
or  after  a  period  of  storage,  and  that  the  chief  end- 
products  are  carbon  dioxid  and  water.  Protein  stands 
somewhat  apart  for  it  yields  compounds  of  nitrogen 
and  sulphur  in  addition  to  the  others. 

We  have  seen  that  a  study  of  the  urine  throws  much 
light  upon  protein  metabolism  but  very  little  upon  that 
of  other  types  of  food.  If  we  are  to  learn  anything 
about  the  quantity  of  carbohydrate  and  fat  subjected 
to  oxidation  we  must  use  an  apparatus  that  will  arrest 
the  carbon  dioxid  escaping  from  the  lungs.  Such  an 
apparatus  is  difficult  to  construct  and  operate,  especially 
if  it  is  on  a  scale  to  deal  with  the  human  body,  but  several 
laboratories  have  been  fully  equipped  for  this  line  of 
work.  Sometimes  it  is  desired  to  include  the  excreta 
of  the  skin  but  often  this  is  unimportant. 

A  short  description  may  be  given  of  what  is  called 
a  respiration  chamber,  a  device  to  collect  the  products  in 
the  expired  air  together  with  those  from  the  skin.  The 
subject  is  confined  in  an  air-tight  compartment.  This 
may  be  in  the  form  of  a  long  box  in  which  we  must  lie 
as  in  a  berth  or  it  may  be  more  spacious.  A  man  has 
remained  for  two  weeks  in  a  chamber  of  the  largest  kind. 
Air  is  pumped  from  the  compartment  at  one  place  and 
returned  at  another.  The  stream  maintained  by  the 
pump  is  treated  as  we  shall  now  explain. 

326 


INCOME    AND    OUTGO 


327 


The  air  which  is  making  the  circuit  outside  the  chamber 
is  first  driven  through  a  jar  containing  sulphuric  acid. 
This  removes  from  it  all  moisture.  The  acid  is  gradu- 
ally diluted  by  the  addition  of  water  brought  from  the 


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chamber  and  gains  weight  in  proportion  as  this  water 
is  added  to  it.  The  weight  of  the  container  is  determined 
at  the  beginning  of  each  experiment  and  by  weighing 
it  again  at  the  close  the  amount  of  water  swept  out  by 
the  air  current  can  be  estimated.  This  water  has  come 


328  HUMAN    PHYSIOLOGY 

partly  from  the  breathing  passages  and  partly  from  the 
skin  of  the  imprisoned  volunteer. 

The  dry  air  from  the  sulphuric  acid  still  contains  the 
carbon  dioxid  which  it  has  gained  through  being  re- 
spired. To  remove  this  it  is  conducted  through  a 
container  in  which  alkali  is  used  to  retain  the  carbon 
dioxid.  Special  precautions  are  taken  to  hold  back  the 
moisture  which  might  be  carried  out  of  the  alkaline 
mass  at  this  point.  The  gain  in  weight  registered  by 
the  alkali  cylinder  during  an  experiment  is  taken  to 
equal  the  output  of  carbon  dioxid.  The  air  thus  freed 
from  water  and  carbon  dioxid  is  returned  to  the  chamber. 

It  will  be  recognized  that  the  scheme,  so  far  as  it  has 
been  described,  keeps  down  the  humidity  and  the  carbon 
dioxid  in  the  air  breathed  by  the  subject  but  does  not 
compensate  for  oxygen  consumed.  This  is  secured  as 
follows:  A  tank  of  pure  oxygen  is  at  hand  and  con- 
nected with  the  chamber.  As  the  oxygen  originally 
available  is  diminished  by  respiration  and  the  result- 
ing carbon  dioxid  is  absorbed  by  the  alkali  the  volume 
of  the  air  in  the  compartment  will  tend  to  contract.  By 
an  automatic  arrangement  any  measurable  shrinkage 
of  the  air  in  the  apparatus  will  automatically  admit 
oxygen  from  the  tank,  while  the  admission  will  cease 
as  soon  as  the  initial  volume  and  density  have  been 
restored. 

The  oxygen  tank  is  weighed  from  time  to  time. 
The  diminution  in  weight  indicates  how  much  oxygen 
has  passed  into  the  chamber  to  take  the  place  of  that 
consumed  by  the  subject.  So,  with  a  chamber  of  this 
pattern,  three  figures  are  obtainable:  the  water  loss,  the 
carbon  dioxid  elimination,  and  the  oxygen  consumption. 
If  the  urine  and  the  feces  are  collected  for  the  same 
period  we  may  consider  that  we  have  a  fairly  complete 
knowledge  of  the  body's  discharges.  Of  course,  the 
task  is  much  less  simple  than  it  has  been  made  to  appear. 
There  are  preliminary  and  subsequent  analyses  of  the 


INCOME    AND    OUTGO  329 

air  in  the  chamber  which  have  to  be  carried  out,  and  there 
are  other  details  which  we  have  not  enumerated. 

The  Energy  of  the  Metabolism. — We  may  look  at  the 
income  and  outgo  of  the  body  from  the  standpoint  of 
energy  as  well  as  from  that  of  matter.  The  food,  looked 
upon  as  fuel,  represents  a  definite  amount  of  potential 
energy.  The  excreta  represent  very  little.  For  any 
discussion  of  the  facts  we  must  have  a  unit  for  the 
measurement  of  energy  and  the  one  most  used  is  the 
large  Calorie.  This  is  primarily  a  heat  unit  but  we  know 
that  all  forms  of  energy  are  convertible  and  we  can  make 
the  Calorie  stand  for  work  or  for  electricity  if  we  choose. 
The  body  generally  disperses  practically  all  its  energy 
in  the  form  of  heat  and  so  the  unit  is  evidently  the  best 
possible.  A  large  Calorie  is  the  amount  of  heat  required 
to  raise  the  temperature  of  a  kilogram  (1000  grams)  of 
water  1°C. 

Oxidizable  compounds  of  a  uniform  composition  are 
said  to  have  definite  fuel  values.  The  fuel  value  of  a 
compound  is  expressed  as  the  number  of  Calories  set 
free  by  completely  oxidizing  1  gram  of  that  compound. 
One  gram  of  sugar  oxidized  to  carbon  dioxid  and  water 
without  by-products  gives  nearly  4  Calories.  Starch 
has  a  value  very  slightly  in  excess  of  that  of  sugar. 
One  gram  of  fat  gives  much  more,  say  9.3  Calories. 
The  fuel  value  of  alcohol  is  about  7  Calories.  These 
figures  do  not  seem  to  vary  whether  the  oxidation  is  a 
literal  burning  or  a  physiologic  decomposition  in  the 
body  cells. 

The  case  of  protein  is  peculiar.  One  gram  of  protein 
burned  in  the  open  with  a  full  supply  of  oxygen  gives 
nearly  6  Calories.  It  contributes  less  than  this  quantity 
of  energy  to  the  organism  for  it  is  less  completely  oxidized 
in  the  life  process  than  it  can  be  by  flame.  Such  products 
as  urea  have  a  moderate  fuel  value  and  represent  energy 
which  the  body  has  failed  to  extract.  They  remind  us 
of  the  cinders  left  by  a  coal  fire.  The  actual  fuel  value 


330  HUMAN    PHYSIOLOGY 

of  protein  to  the  body  is  a  trifle  over  4  Calories.  It  is 
nearly  the  same  as  that  of  the  carbohydrates. 

Calorimetry. — A  calorimeter  is  an  apparatus  for 
measuring  the  evolution  of  heat.  It  may  be  adapted 
to  show  how  much  heat  is  given  off  by  samples  of  food 
or  other  substances  when  burned  in  oxygen.  It  may 
be  of  a  form  to  hold  a  living  animal  or  even  a  man  and 
to  give  us  data  in  regard  to  the  evolution  of  heat  that 
accompanies  the  metabolic  process.  A  respiration 
chamber  such  as  has  been  described  may  be  a  calori- 
meter also.  When  it  has  this  feature  we  can  obtain 
simultaneously  the  material  and  the  dynamic  output 
of  the  inmate. 

In  some  of  the  early  and  crude  attempts  to  find  out 
how  much  heat  issues  from  the  body  of  an  animal  the 
estimates  were  based  upon  observing  how  much  ice 
could  be  melted  at  the  expense  of  the  metabolism.  This 
involved  chilling  the  animal  and  has  been  abandoned 
in*  favor  of  better  methods.  Sometimes  the  heat  is 
reckoned  by  recording  the  rise  of  temperature  in  a 
volume  of  air  which  is  exposed  to  the  animal's  influence. 
In  other  cases  the  heat  is  absorbed  in  a  known  mass  of 
water  and  the  calculation  based  on  the  extent  to  which 
the  temperature  is  raised.  This  is  the  principle  followed 
in  the  great  calorimeters  applicable  to  the  human  subject. 

It  is  important  to  understand  that  not  all  the  energy 
produced  in  the  course  of  the  metabolism  results  in  heat 
that  is  directly  measurable.  The  most  considerable 
exception  is  found  in  the  disappearance  or  making 
" latent"  of  a  large  quantity  of  heat  through  the  evapo- 
ration of  water.  A  man  in  a  respiration  chamber  may 
easily  evaporate  a  liter  (1000  grams)  of  water  in  twenty- 
four  hours.  This  change  in  so  large  a  mass  of  water 
from  the  liquid  to  the  gaseous  state  entails  the  ap- 
parent annihilation  of  heat  to  the  amount  of  more 
than  500  Calories.  (It  is  not  really  annihilation,  how- 
ever; the  heat  becomes  tangible  again  when  the  vapor 
is  condensed.) 


INCOME    AND    OUTGO  331 

A  man  in  a  calorimeter,  therefore,  gives  out  a  certain 
quantity  of  heat  which  can  be  deduced  from  the  warm- 
ing effect  on  water  circulating  in  coils  and  he  is  credited 
with  having  produced  an  additional  .quantity  which  is 
estimated  by  ascertaining  how  much  water  has  evapor- 
ated from  his  skin  and  respiratory  tract.  We  have  said 
that  the  latter  quantity  may  be  as  much  as  500  Calories. 
The  total  will  probably  approach  2000  Calories  for  a 
resting  adult.  If  the  body  temperature  is  not  the  same 
at  the  end  of  the  experimental  period  that  it  was  at 
the  beginning  a  correction  has  to  be  made  for  heat 
retained  or  dissipated,  as  the  case  may  be.  For  example, 
if  the  body  is  equivalent  to  50  kilograms  of  water  as  a 
container  of  heat,  and  its  temperature  has  risen  0.5°C. 
during  the  experiment,  25  Calories  must  have  been  stored 
in  it.  This  must  be  added  to. the  other  quantities  to 
arrive  at  a  correct  estimate  of  the  heat  produced. 

The  total  daily  production  of  heat  by  a  full-grown  man 
is  not  likely  to  fall  below  1500  Calories  under  any  cir- 
cumstances that  can  be  called  normal.  The  maximum 
is  in  the  vicinity  of  10,000  Calories.  These  limits  are 
so  widely  separated  that  one  seeks  at  once  for  a  de- 
termining condition  and  will  probably  draw  a  correct 
inference  as  to  what  it  is.  No  other  factor  influencing 
metabolism  approaches  in  importance  muscular  activity. 
The  heat  production  is  nearly  proportional  to  the  work 
performed.  It  is  also  true  that  the  discharge  of  carbon 
dioxid  varies  in  the  same  sense.  Since,  as  we  have 
already  found,  the  excretion  of  nitrogen  has  no  such  rise 
and  fall  we  have  here  clear  evidence  in  support  of  a 
previous  general  assertion:  that  muscular  contractions 
are  made  at  the  expense  of  non-nitrogenous  fuel. 

Someone  may  raise  the  question,  does  the  calorimeter 
give  credit  in  Calories  for  energy  expended  in  the  per- 
formance of  mechanical  work?  It  may  be  answered 
that  it  does  excepting  under  special  conditions.  Sup- 
pose, for  instance  that  the  subject  of  an  experiment 
engages  in  sawing  wood.  The  ultimate  result  of  his 


332  HUMAN    PHYSIOLOGY 

efforts  is  merely  heat  production  through  friction.  Or 
take  the  case  of  his  heart:  this  organ  impresses  energy 
upon  the  blood  but  it  is  all  turned  back  to  heat  as  the 
resistance  of  the  vessels  is  overcome.  In  the  same  way, 
the  work  done  by  the  breathing  muscles  is  rendered  to 
the  calorimeter  as  heat,  for  the  weight  which  is  lifted 
is  allowed  to  sink  back  again  at  every  expiration  and 
there  is  no  storage  of  potential  energy. 

If  the  man  in  the  chamber  should  operate  a  force 
pump  and  permanently  elevate  a  quantity  of  water  to 
a  tank  overhead  some  of  his  energy  would  actually  fail 
to  appear  as  heat.  This  would  illustrate  one  of  the 
"special  conditions"  referred  to  in  the  preceding  para- 
graph. The  general  principle  is  that  a  man  in  a  calo- 
rimeter will  receive  credit  for  all  the  energy  he  expends 
in  muscular  work,  provided  that  he  does  not  produce 
lasting  changes  in  his  environment. 

Indirect  Calorimetry. — If  a  respiration  chamber  has 
not  the  appliances  to  make  it  a  calorimeter  it  is  still 
possible  to  estimate  the  energy  production  of  the  captive. 
We  can  calculate  from  his  material  output  how  much  of 
the  standard  fuel  substances  he  has  decomposed  and 
then  we  can  assign  to  these  their  recognized  calorific 
values.  This  procedure  is  known  as  indirect  calorimetry. 
It  will  be  worth  while  to  give  a  rough  idea  of  how  this 
is  done. 

Our  data  are  (1)  the  nitrogen  excretion;  (2)  the  carbon 
loss,  and  (3)  the  oxygen  consumption.  We  do  not  need 
to  know  the  water  outgo  in  this  case.  The  calculation 
of  the  protein  metabolism  from  the  nitrogen  is  a  step  we 
have  already  taken.  We  will  assume,  as  once  before, 
that  the  protein  decomposed  amounted  to  75  grams. 
This  should  have  furnished  a  trifle  .more  than  300  Calories, 
The  carbon  loss  has  been  mainly  in  the  respiratory 
carbon  dioxid  but  our  total  must  include  the  small 
quantity  in  the  urinary  compounds.  Suppose  that 
the  total  is  250  grams.  We  must  deduct  from  it  the 
car.bon  in  75  grams  of  protein,  about  39  grams.  The 


INCOME    AND    OUTGO  333 

remainder,  211  grams,  stands  for  carbon  from  carbo- 
hydrate and  fat. 

One  would  say  that  we  had  now  come  face  to  face 
with  a  hopeless  difficulty.  How  can  we  assign  to  each 
of  the  two  types  of  non-nitrogenous  fuel  the  proper  share 
of  the  carbon?  There  is  nothing  individual  about  their 
end-products.  The  division  of  the  carbon  into  these 
portions  is,  in  fact,  too  difficult  to  explain  in  detail. 
But  the  means  of  guidance  is  furnished  by  the  oxygen 
consumption.  The  amount  necessary  to  oxidize  a 
gram  of  fat  is  different  from  that  needed  to  oxidize  a 
gram  of  sugar.  When  a  certain  quantity  of  oxygen  is 
known  to  have  been  used  to  release  a  certain  quantity 
of  carbon  dioxid  the  expert  in  nutrition  has  no  trouble 
in  solving  equations  that  show  how  much  fat  and  how 
much  sugar  have  been  used. 

Let  us  suppose  that  in  our  hypothetic  example  the 
211  grams  of  carbon  from  non-protein  sources  is  found 
to  represent  91  grams  of  fat  and  350  grams  of  carbo- 
hydrate. (These  are  possible  figures.)  We  can  go 
on  to  say  that  91  grams  of  fat  should  have  given  rise 
in  its  decomposition  to  about  845  Calories,  while  350 
grams  of  carbohydrate  should  have  yielded  about 
1400  Calories.  Adding: 

Calories  from  protfein 300 

Calories  from  fat ; 845 

Calories  from  carbohydrate 1400 


Total 2545 

When  a  man  is  in  a  calorimeter  which  is  also  a  respira- 
tion chamber  we  have  the  interesting  possibility  of 
comparing  the  heat  which  he  actually  produces  with 
that  which  he  should  theoretically  evolve — direct  with 
indirect  calorimetry. 

The  figures  obtained  are  in  close  agreement.  This 
means  that  the  living  organism  produces  no  measurable 
energy  which  cannot  be  accounted  for  as  derived  from 


334  HUMAN    PHYSIOLOGY 

the  consumption  of  fuel.  It  is,  in  effect,  the  demon- 
stration that  the  body  is  strictly  subject  to  the  principle 
of  the  conservation  of  energy.  An  alcohol  lamp  can 
be  substituted  for  a  man  and  the  calculations  of  direct 
and  indirect  calorimetry  carried  out  upon  it  with  parallel 
results.  If  we  had  found  that  a  man  could  produce  3000 
Calories  when  his  fuel  consumption  entitled  him  to  only 
2000  we  should  have  concluded  that  the  living  state  con- 
ferred upon  matter  the  ability  to  create  as  well  as  to 
transform  energy. 

The  fact  is  to  be  pressed  home  that  we  can  judge  of 
the  metabolism  with  great  accuracy  by  analyzing  the 
excreta  but  not  by  fixing  our  attention  upon  the  day's 
ration.  There  is  a  presumption  that  the  food  will 
correspond  in  quantity  and  proportion  with  the  material 
metabolized  but  this  is  only  a  presumption.  We  have 
only  to  consider  that  the  income  may  be  nil  and  the 
metabolism  large,  as  when  a  starving  fisherman  rows 
for  his  life.  Over  long  periods  there  must,  of  course, 
be  an  approximation  of  the  diet  to  the  metabolism. 
The  observation  that  the  daily  ration  of  a  Maine  lumber- 
man has  a  value  of  7000  Calories  or  more  is  a  fair  indi- 
cation that  the  output  of  energy  is  of  the  same  order, 
but  we  cannot  assume  a  close  correspondence  upon  a 
single  day. 

There  have  been  many  studies  of  the  diets  chosen  by 
various  classes  of  people.  Men  who  do  hard  work 
commonly  secure  a  supply  of  3500  Calories  or  there- 
abouts. This  is  the  figure  for  farmers  in  widely  sepa- 
rated countries.  The  ration  of  sedentary  individuals 
may  be  2500  Calories  or  even  less.  Advocates  of  low 
feeding  have  urged  that  these  allowances  are  too  liberal 
and  may  be  reduced  with  advantage  by  500  Calories 
or  so.  It  does  not  appear  that  many  people  have  de- 
parted greatly  from  the  average  practice  of  the  race, 
unless  for  rather  short  periods.  We  shall  refer  to  this 
question  again. 

When  we  speak  of  the  calorific  value  of  a  ration  we 


INCOME    AND    OUTGO  335 

are  assuming  perfect  absorption.  Of  course  this  in- 
volves an  error  but  not  a  large  one.  One  often  hears 
the  suggestion  that  a  certain  person  "gets  more  of  the 
goodness  from  his  food"  than  does  another.  There  is 
little  evidence  in  support  of  this  judgment  unless  the 
second  individual  is  a  victim  of  chronic  diarrhea.  Lec- 
turers and  writers  may  carelessly  assert  that  it  is  possible 
by  some  system — prolonged  mastication,  it  may  be — 
to  improve  the  assimilation  "by  50  per  cent."  or  "100 
per  cent."  The  ordinary  efficiency  of  the  digestive 
system  is  such  that  an  improvement  of  3  or  4  per  cent, 
is  the  most  that  is  even  conceivable. 

Proportions  of  the  Foods. — It  has  been  said  that  the 
free  selection  of  foods  to  supply  the  needs  of  the  day 
is  likely  to  lead  to  the  inclusion  of  about  75  grams  of 
protein.  The  foods  chosen  will  usually  give  about  the 
proper  number  of  Calories,  and  since  75  grams  of  protein 
furnish  a  little  over  300  it  will  be  seen  that  an  average 
of  about  2000  Calories  must  be  represented  by  carbo- 
hydrate and  fat.  The  fat  in  freely  selected  food  will 
not  often  exceed  100  grams  a  day.  It  will  in  many  cases 
be  much  less,  perhaps  no  more  than  50  grams.  The 
calories  from  50  grams  of  fat  will  be  about  465,  from  100 
grams  about  930.  In  round  numbers,  then,  the  carbo- 
hydrates of  the  ration  must  furnish  from  1000  to  1500 
Calories.  From  250  to  375  grams  of  starch  and  sugar 
will  answer  the  purpose. 

The  following  combination  may  be  considered  a 
common  type  of  ration: 

Protein 75  grams  or    300  Calories. 

Fat 50  grams  or    465  Calories. 

Carbohydrate 375  grams  or  1500  Calories. 


Total 2265  Calories. 

An  indefinite  number  of  combinations  can  be  suggested 
in  which  the  protein  and  the  total  Calories  shall  be 
constant.  If,  for  example,  the  fat  is  increased  to  100 
grams  we  may  have  the  items  as  below: 


336  HUMAN    PHYSIOLOGY 

Protein 75  grams  or    300  Calories. 

Fat 100  grams  or    930  Calories. 

Carbohydrate 250  grams  or  1000  Calories.. 

Total 2230  Calories. 

We  have  not  been  at  pains  to  keep  the  total  precisely 
the  same.  We  are  using  only  approximate  figures  for 
the  heat  values  of  protein  and  carbohydrate  so  that 
there  would  be  no  point  in  refining  other  parts  of  the 
calculation. 

Once  more,  there  is  the  possibility  of  the  inclusion 
of  alcohol  in  the  diet  with  some  reduction  of  the  other 
non-nitrogenous  foods.  This  may  now  be  indicated: 

Protein 75  grams  or  300  Calories. 

Fat 90  grams  or  837  Calories. 

Carbohydrate 230  grams  or  920  Calories. 

Alcohol 25  grams  or  175  Calories. 


Total 2232  Calories. 

When  a  ration  is  increased  to  meet  the  requirements 
of  heavy  muscular  work  there  is  not  likely  to  be  any 
great  addition  to  the  protein  fraction.  The  extra 
Calories  are  obtained  from  fats  and  carbohydrates,  the 
latter  bearing  the  brunt  of  the  demand.  Among  the 
most  surprising  statistics  of  diet  are  those  regarding  the 
huge  intake  of  patients  fighting  tuberculosis.  The 
daily  protein  may  exceed  200  grams,  the  fat  300,  and 
the  carbohydrate  500,  with  energy  totals  in  the  vicinity 
of  5000  Calories.  The  wasting  of  the  tissues  in  this 
disease  can  only  be  compensated  by  a  protein  supply 
which  in  health  would  be  considered  most  unhygienic. 

Conditions  Affecting  Metabolism. — We  have  rightly 
given  the  foremost  place  among  the  factors  that  de- 
termine the  amount  of  the  metabolism  to  work.  When 
we  say  that  exposure  to  cold  greatly  increases  the  oxida- 
tion it  might  seem  that  we  were  introducing  another 
factor,  but  we  are  really  instancing  what  is  merely  a 


INCOME    AND    OUTGO  337 

special  case  of  muscular  activity.  The  simple  fact  is 
that  cold  leads  to  an  increase  in  reflex  tonus,  to  shiver- 
ing, and  to  instinctive  briskness  of  movement.  The 
result  is  thus  a  form  of  exercise. 

The  change  from  fasting  to  feeding  influences  the 
metabolism  much  less  than  might  be  anticipated.  It 
is  true  that  a  fasting  animal  will  usually  be  found  to 
have  a  low  metabolism  but  this  is  because  it  is  disposed 
to  be  quiet.  If  it  is  given  food,  and  remains  as  inactive 
as  before,  the  increase  in  oxidation  will  be  very  moderate. 
It  is  not  likely  to  exceed  20  per  cent.,  whereas  mild 
muscular  activity  will  double  the  original  quantity. 
The  tendency  of  protein  food  to  increase  the  metabolism 
is  much  more  marked  than  that  of  the  non-nitrogenous 
types. 

Changes  in  the  state  of  the  tissues  and  fluids  of  the 
body  as  a  result  of  the  altered  activity  of  certain  organs 
may  considerably  change  the  total  metabolism.  This 
is  particularly  true  of  the  thyroid  gland,  a  structure 
which  will  be  more  fully  dealt  with  by  and  by.  The 
general  statement  may  be  made  here  that  abnormal 
activity  of  the  thyroid  means  a  high  rate  of  decomposi- 
tion among  the  stores  of  the  body.  Accordingly,  people 
with  overactive  thyroids  become  emaciated  and  the 
extract  of  the  organ  is  often  used  in  patent  medicines 
intended  to  reduce  weight.  Deficient  thyroid  activity 
may  help  to  cause  obesity  in  certain  cases,  the  oxidation 
in  the  tissues  lagging  behind  the  assimilation  of  new 
material. 

Small  animals  have  a  greater  average  metabolism, 
weight  for  weight,  than  larger  ones.  This  is  true  whether 
we  have  reference  to  the  young  and  adults  of  one  species 
or  to  the  adults  of  two  species,  as  mice  and  rats.  It 
has  been  shown  that  the  metabolism  is  more  nearly 
proportional  to  surface  than  to  weight.  It  is  a  principle 
of  geometry  that  the  smaller  one  of  two  solids  of  similar 
form  has  more  surface  in  proportion  to  its  mass  than 
the  larger.  If  a  mouse  is  half  as  long  as  a  rat  and  of 

22 


338 


HUMAN    PHYSIOLOGY 


the  same  build  it  will  weigh  about  one-eighth  as  much, 
while  it  will  have  one-fourth  the  surface  of  the  larger 
animal.  Its  metabolism  under  comparable  conditions 
will  be  nearer  one-fourth  than  one-eighth  that  of  the 
rat. 

So  a  baby  a  year  old  and  weighing  20  pounds  will 
probably  have  a  metabolism  of  more  than  twice  as 
many  Calories  per  unit  of  weight  as  that  exhibited  by 
its  parents.  The  rate  will  be  fairly  uniform  for  both 
when  it  is  referred  to  surface  area.  Such  comparisons 
are  made  at  an  external  temperature  high  enough  to 


a 

FIG.  68. — If  a  cube  measures  twice  as  much  as  another  along  one  of 
its  edges,  it  will  have  four  times  the  surface  and  eight  times  the  weight 
of  the  smaller.  The  same  principle  must  apply  to  two  animals  if  they 
are  alike  in  build. 

avoid  the  reflex  stimulation  of  the  skeletal  muscles  to 
which  we  have  alluded.  The  area  of  the  full-grown 
human  body  has  been  estimated  at  1.7  square  meters, 
and  as  the  minimum  metabolism  is  somewhat  less  than 
2000  Calories  the  standard  quantity  per  square  meter 
appears  to  be  about  1000,  a  convenient  round  number.1 
It  does  not  vary  much  from  this  in  several  very  unlike 
species  of  animals  of  the  "  warm-blooded "  order. 

Students  are  likely  to  question  whether  mental  exer- 
tion noticeably  increases  the  metabolism.  Careful  ob- 

1  According  to  certain  authorities  the  surface  is  greater,  about  2 
square  meters.  This  gives  a  daily  value  per  square  meter  of  800 
Calories  or  a  little  more. 


INCOME    AND    OUTGO  339 

servations  have  been  made  upon  this  point  and  it  may 
be  said  that  any  such  increase  is  trifling.  It  can  prob- 
ably be  said  further  that  an  increase  noted  in  a  man 
who  is  using  his  cerebrum  is  not  due  to  extra  oxidation 
in  the  gray  matter  but  to  a  tension  of  the  muscles  un- 
consciously developed.  We  must  bear  in  mind  that 
the  brain  makes  only  about  2.5  per  cent,  of  the  total 
mass  of  the  body,  and  that  only  a  small  fraction  of  it 
can  be  supposed  to  be  the  seat  of  the  special  activity 
attending  intellectual  work.  Emotional  excitement 
means  a  great  increase  in  metabolism,  but  here  again 
it  is  not  cerebral  oxidation  which  is  registered,  but 
the  secondary  activity  evoked  in  muscles  and  glands. 


CHAPTER  XXV 
THE  REQUISITES  OF  THE  DIET 

In  this  chapter  we  shall  include  a  summary  and 
restatement  of  much  that  has  gone  before.  We  shall 
also  have  new  material  to  add.  If  we  begin  the  attempt 
to  define  an  adequate  diet  we  shall  probably  think  first 
of  the  calorific  requirement.  If  the  specific  case  is 
that  of  a  student  taking  a  limited  amount  of  exercise 
we  may  fix  upon  2500  Calories  as  a  fair  standard.  We 
must  think  next  of  the  protein  allowance. 

The  amount  mentioned  in  the  last  chapter,  75  grams, 
is  about  what  one  is  likely  to  take.  The  older  state- 
ment that  the  average  protein  ration  is  100  grams  a 
day  is  not  confirmed  by  recent  reports.  The  protein 
should  be  from  several  sources.  The  unequal  nutritive 
virtues  of  different  proteins  should  be  recalled.  Pro- 
teins for  growth  may  have  ceased  to  be  called  for,  but 
proteins  for  maintenance  must  still  be  supplied. 

The  non-protein  part  of  the  diet  will  be  composed  of 
carbohydrates  and  fats  in  proportions  which  may  be 
widely  varied.  The  usual  tendency  is  to  take  both,  but  to 
make  the  greater  use  of  carbohydrates.  The  Esquimaux 
exhibit  the  possibility  of  living  on  proteins  and  fats  with 
a  minimum  of  starch  and  sugar,  but  this  course  is  forced 
upon  them  by  necessity  and  does  not  indicate  that  it 
would  be  chosen  instinctively.  Their  protein  consump- 
tion is  very  great  and  we  must  remember  that  surplus 
protein  becomes  a  source  of  sugar  in  the  system.  Since 
sugar  must  be  at  hand  when  fat  is  to  be  oxidized  we  may 
suppose  that  carnivorous  men  and  animals  are  pro- 
tected against  acidosis  because  they  take  more  protein 
than  they  appear  to  need. 

340 


THE    REQUISITES    OF    THE    DIET  341 

At  the  other  extreme  from  the  Esquimaux  we  have 
the  vegetarians  of  various  classes.  They  are  not  likely 
to  eat  too  much  protein  and  they  are  sure  of  plenty  of 
carbohydrate.  When  their  diets  are  scientifically 
planned  they  often  contain  a  goodly  amount  of  fat 
secured  by  the  use  of  nuts  and  vegetable  oils.  If  milk 
and  eggs  are  permitted  the  protein  and  fat  can  be  more 
readily  brought  up  to  the  standards  of  mixed  diet.  Eggs 
are  like  meat  in  their  very  low  carbohydrate  content. 

Alcohol  enters  with  considerable  regularity  into  the 
diet  of  many.  This  is  particularly  true  in  Europe.  As 
has  been  indicated  before,  it  acts  much  like  other  non- 
nitrogenous  food.  According  to  a  careful  estimate, 
about  one-fifth  of  the  calorific  requirement  of  the  day 
may  be  met  by  alcohol  without  producing  any  sugges- 
tion of  intoxication.  That  is  to  say,  if  2500  Calories 
is  the  total  called  for,  some  500  may  be  represented 
by  alcohol.  This  means  about  70  grams  of  the  pure 
compound,  an  amount  which  might  be  conveyed  in 
less  than  half  a  pint  of  whiskey,  a  full  pint  of  a  fortified 
wine  like  port,  a  quart  of  a  light  wine,  or  3  quarts  of 
beer.  To  say  that  it  is  possible  to  use  alcohol  to  such 
an  extent  is  by  no  means  to  advise  it. 

Isodynamic  Quantities. — From  what  has  been  said 
it  will  be  gathered  that  carbohydrate  and  fat  are  in  a 
great  degree  interchangeable  and  that  alcohol  can  be 
substituted  for  either  within  certain  limits.  We  must 
point  out  that  when  substitutions  of  this  kind  are  made 
the  proportion  is  not  gram  for  gram,  but  determined 
by  calorific  values.  Thus  a  gram  of  fat  has  the  available 
energy  of  about  2^  grams  of  sugar.  On  the  same  prin- 
ciple it  can  replace  1^  grams  of  alcohol.  Quantities  of 
different  foods  which  have  equal  fuel-values  are  said  to 
be  isodynamic. 

Water. — The  services  of  water  to  the  system  should 
be  evident  by  this  time.  It  is  necessary,  first  of  all, 
to  the  normal  constitution  of  the  tissues.  It  makes  up 
about  75  per  cent,  of  most  of  them  and  about  65  per 


342  HUMAN    PHYSIOLOGY 

cent,  of  the  whole  body.  (The  presence  of  the  bones 
somewhat  lowers  the  average.)  These  proportions 
cannot  be  much  altered  by  the  severest  drafts  upon  the 
water  of  the  organism.  When  it  is  reduced,  even  by 
a  little;  the  secretions  are  diminished  and  thirst  becomes 
imperious.  It  is  plain  from  these  facts  that  water  is 
necessary  to  growth;  it  is  as  essential  to  new  tissue  as 
protein  itself. 

Water  is  necessary  to  maintenance  as  well  as  to 
growth  because  it  must  make  good  the  losses  from  the 
body.  These  losses  cannot  be  checked  because  the 
urinary  excretion  must  have  water  as  its  vehicle  and 
because  much  of  the  time  water  must  be  evaporated 
from  the  skin  and  the  breathing  passages  to  remove  an 
excess  of  heat.  The  secretion  through  the  skin  falls 
to  a  low  level  when  there  is  no  call  for  it,  but  there  seems 
to  be  no  way  to  keep  back  the  water  that  is  taken  by  the 
respired  air.  This  is  a  secretion  which  is  never  sup- 
pressed and,  unlike  most  others,  it  is  one  which  environ- 
mental conditions  directly  determine,  the  nervous 
system  seeming  not  to  regulate  it  at  all. 

Mineral  Matter. — There  is  abundant  evidence  to 
show  that  living  protoplasm  cannot  exist  without 
mineral  as  well  as  organic  constituents.  It  may  be  that 
while  the  living  state  continues  these  mineral  substances 
— salts  of  sodium,  potassium,  calcium,  etc. — are  united 
with  the  proteins  in  temporary  combinations.  In  fact, 
the  materials  which  we  call  proteins  as  we  handle  them 
in  the  laboratory  are  not  often  "  ash-free."  This  means 
that  they  leave  mineral  residues  when  they  are  burned. 
Many  experiments  have  proved  that  the  thorough  re- 
moval of  ash  from  the  food  given  to  animals  impairs 
its  nutritive  value. 

The  need  of  a  mineral  ration  is  naturally  more  distinct 
when  growth  is  in  progress  than  it  is  later.  But  it  is 
usually  stated  that  it  never  entirely  ceases.  The  ash 
of  milk  has  been  laboriously  analyzed  and  it  has  been 
found  to  show  a  marvelous  fitness  for  the  purpose  it 


THE    REQUISITES    OF    THE    DIET  343 

must  serve.  Milk  is  made  by  the  cells  of  the  mammary 
gland.  The  antecedent  materials  are  presented  to 
these  cells  by  the  blood  through  the  medium  of  the 
lymph.  Yet  the  selective  power  of  the  gland  tissue 
is  such  that  the  milk  is  radically  unlike  the  blood  in 
almost  all  its  characters. 

This  is  most  strikingly  true  of  the  salts.  In  blood 
the  leading  saline  compound  is  sodium  chlorid.  In 
milk  this  salt  is  sparingly  present  and  calcium  phosphate 
stands  first.  The  lime  has  been  derived  from  blood  in 
which  it  occurs  in  only  a  minute  percentage.  While 
the  ash  of  milk  is  utterly  unlike  that  of  blood  or  blood 
plasma,  it  is  remarkably  like  the  ash  obtained  by  cremat- 
ing the  young  animal  entire.  The  high  percentage  of 
lime  in  milk  answers  to  the  need  for  developing  a 
skeleton.  The  gland  is  not  compelled  to  deliver  a 
filtrate  such  as  might  be  anticipated  from  a  study  of 
the  plasma,  but  is  able  to  prepare  a  secretion  prophetic 
of  a  body  which  does  not  yet  exist. 

One  discrepancy  between  the  ash  of  milk  and  that 
of  the  young  animal  has  been  reported:  a  deficiency  in 
iron.  But  it  has  been  shown  at  the  same  time  that 
new-born  animals  have  more  iron  for  their  weight  than 
they  will  have  later.  It  is  thus  normal  for  them  to 
receive  a  food  that  is  poor  in  this  element  for  a  while. 
Iron,  by  the  way,  is  one  of  the  indispensable  elements. 
We  cannot  have  hemoglobin  without  it  and  it  is  doubt- 
less needed  in  other  connections.  It  is  best  utilized  when 
it  is  taken  in  the  form  of  complex  organic  combinations. 
Among  the  foods  which  furnish  it  in  appreciable  amounts 
are  meats,  yolk  of  eggs,  apples,  strawberries,  asparagus, 
and  spinach. 

Sodium  Chlorid. — While  food  ordinarily  contains  a 
great  variety  of  mineral  compounds  there  is  one  which 
we  deliberately  add  to  our  diet.  This  is  common  or 
table  salt  (sodium  chlorid).  As  much  as  10  pounds 
may  be  eaten  in  a  year  by  a  man  of  average  tastes. 
We  hear  of  people  who  are  said  not  to  eat  any,  but  such 


344  HUMAN    PHYSIOLOGY 

individuals  must  receive  a  moderate  quantity  even 
though  they  aim  to  avoid  it;  it  is  represented  in  most 
foods.  The  bulk  of  the  salt  which  we  eat  is  taken  to 
improve  the  flavor  of  various  dishes,  but  a  certain  amount 
seems  to  be  a  more  fundamental  necessity. 

World-wide  observation  has  shown  that  salt  is  gen- 
erally prized  by  the  vegetarian  races  but  disliked  by 
those  that  are  approximately  carnivorous.  The  animals 
which  seem  to  crave  it  are  herbivorous,  cattle,  sheep, 
and  deer  being  among  them.  Cats  and  dogs  show  an 
aversion  to  meat  that  is  noticeably  salt.  An  explanation 
of  these  divergent  instincts  has  been  offered  which  is 
ingenious  and  apparently  reasonable. 

Salts  of  sodium  and  potassium  are  mingled  in  natural 
foods.  In  most  cases  the  potassium  compounds  are 
in  excess,  but  sometimes  their  preponderance  is  only 
moderate,  while  in  other  cases  it  is  overwhelming.  In 
meat  the  potassium  does  not  so  greatly  exceed  the  sodium 
as  it  does  in  most  vegetables.  Potatoes,  for  example, 
are  very  rich  in  potassium  and  poor  in  sodium.  It  is 
argued  that  if  the  system  is  loaded  with  a,  large  quantity 
of  potassium  salts  the  duty  of  restoring  the  composition 
of  the  blood  to  the  normal  standard  must  fall  upon  the 
kidneys.  These  organs  will  soon  eliminate  the  surplus 
potassium,  but  while  they  are  doing  it  they  will  let  slip 
more  or  less  of  the  abundant  sodium  chlorid  of  the 
plasma.  A  definite  need  for  this  salt  will  then  arise. 

An  investigator  has  shown  that  eating  much  potas- 
sium has  just  the  effect  assumed.  Experimenting  upon 
himself  he  swallowed  salts  of  potassium  until  he  had 
taken  on  a  single  day  the  equivalent  of  18  grams  of 
potassium  oxide.  All  this  soon  passed  into  the  urine, 
but  there  was  lost  with  it  7  grams  of  sodium  chlorid 
for  which  the  diet  made  no  compensation.  The  total 
potassium  intake  in  this  experiment  seems  large,  but  it 
was  not  greater  than  might  be  ingested  in  a  day  by  one 
living  chiefly  upon  potatoes. 

Stefansson,   the   Arctic   traveller,   has   remarked   the 


THE    REQUISITES    OF    THE    DIET  345 

dislike  for  salt  exhibited  by  the  carnivorous  Esquimaux. 
When  he  first  settled  among  them  he  was  embarrassed 
by  the  demand  that  he  should  provide  food  for  all 
comers.  This  was  the  social  convention  and  he  did 
not  wish  to  violate  it  though  his  stores  were  threatened 
with  rapid  depletion.  However,  he  found  a  happy 
solution  of  the  problem:  if  he  salted  the  food  very  moder- 
ately, merely  to  his  own  liking,  his  guests  were  content 
with  a  little.  The  requirements  of  hospitality  were 
met  and  the  provisions  were  conserved. 

Organic  Extractives. — We  recognize  in  our  food  many 
minor  substances  mixed  with  the  proteins,  carbohydrates, 
fats,  water,  and  mineral  salts.  These  are  called  ex- 
tractives or  food  accessories.  Together  with  the  salts 
they  are  responsible  for  most  of  the  flavor  of  different 
foods  and  for  nearly  all  the  odors  which  make  or  mar 
our  meals.  Pure  proteins,  starches,  and  fats  are  prac- 
tically tasteless.  The  sweetness  of  the  sugars  is  about 
the  only  taste  that  can  be  discovered  in  the  absence  of 
the  salts  and  extractives.  Even  the  sugars  and  salts 
are  odorless,  so  we  have  to  conclude  that  the  subtle  and 
attractive  qualities  that  differentiate  between  foods 
and  appeal  so  strongly  to  the  appetite  are  dependent  on 
substances  that  we  know  little  about  and  which  form 
but  a  trifling  total  by  weight  in  any  diet. 

We  have  learned  that  it  is  most  important  that  food 
shall  be  appetizing.  It  is  not  mere  gratification  that 
is  secured,  but  more  efficient  work  on  the  part  of  the 
digestive  mechanisms  both  motor  and  secretory.  En- 
joyment of  a  meal  is  generally  a  guarantee  of  good  diges- 
tion unless  the  circumstances  are  quite  unusual.  We 
ought  to  eat  what  we  like — though  it  may  be  suggested 
that  it  is  desirable  to  extend  the  range  of  our  liking. 
We  may  be  able  to  worry  down  something  that  does 
not  strike  us  very  favorably  and  let  it  be  digested  by  the 
juices  that  have  flowed  primarily  in  response  to  some 
more  delectable  article.  Certain  food  accessories  seem 


346  HUMAN    PHYSIOLOGY 

to  deserve  the  designation  of  secretagogues  already  used 
in  Chapter  XIV. 

Meat  is  peculiarly  rich  in  extractive  bodies.  Among 
these  are  the  ones  which  are  most  surely  capable  of 
arousing  the  gastric  glands  to  activity.  On  the  other 
hand,  the  extractives  of  meat  are  often  condemned  as 
adding  needlessly  to  the  work  of  the  kidneys.  Some 
of  them  are  formers  of  uric  acid,  and  it  is  desirable  in 
many  cases  to  keep  the  production  of  this  refractory 
waste-product  at  the  lowest  possible  level.  Opponents 
of  meat  claim  that  these  extractives  are  drug-like 
compounds  tending  to  establish  an  irritable  disposition 
and  to  diminish  resistance  to  fatigue. 

The  stimulating  principle  in  tea  and  coffee  (caffein) 
is  chemically  allied  to  some  of  the  extractives  of  meat. 
But  it  does  not  appear  to  give  rise  to  uric  acid  and  it 
has  been  shown  to  increase  working  power  in  a  definite 
degree.  The  substance  theobromin  in  chocolate  and 
cocoa  is  related  to  caffein  in  composition  and  action  but 
is  rather  milder  in  most  of  its  effects.  It  is  natural 
to  suppose  that  any  stimulant  which  favors  a  rapid 
expenditure  of  energy  for  a  time  will  also  induce  a  period 
of  depression  and  sluggishness  as  a  reaction.  So  it  is 
a  surprise  to  learn  that  the  most  skilled  investigators 
have  not  detected  any  fall  below  the  average  normal 
condition  when  the  initial  effects  of  caffein  have  passed 
off. 

Vitamins. — The  extractives  we  have  been  discussing 
have  been  such  as  are  favorable  to  some  phases  of 
physiologic  activity  but  not  indispensable.  It  has 
lately  become  probable  that  certain  substances  of  this 
class  are  truly  vital  in  their  importance — that  the  nutri- 
tion of  the  body  cannot  be  maintained  without  them. 
The  name  vitamin  has  been  proposed  for  any  such  com- 
pound. An  amin  is  a  nitrogenous  body  of  a  certain 
molecular  type  and  the  prefix  emphasizes  the  idea  that 
the  one  referred  to  is  necessary  to  life.  We  do  not  know 
how  many  compounds  there  are  which  deserve  to  rank 


THE    REQUISITES    OF    THE    DIET  347 

as  vitamins.  The  list  will  probably  be  extended  year 
by  year. 

Deficiency  Diseases. — Long  before  the  conception 
of  vitamins  was  clearly  presented  it  was  known  that 
nutritional  disasters  sometimes  resulted  from  restricted 
and  peculiar  diets.  The  records  of  explorers  contained 
many  references  to  scurvy,  a  distressing  disease  which 
prostrated  many  members  of  their  parties  when  the 
food  was  limited  in  variety  and  not  fresh.  The  victims 
of  scurvy  became  very  weak  and  suffered  from  intense 
soreness  of  the  gums,  loosening  of  the  teeth,  a  tendency 
to  hemorrhage,  friability  of  the  bones,  and  other  symp- 
toms. Certain  articles,  such  as  lemons,  limes,  potatoes, 
and  fresh  meat,  gained  the  reputation  of  being  anti- 
scorbutics, that  is,  of  preventing  or  curing  scurvy. 

It  was  formerly  held  that  the  foods  causing  scurvy 
had  become  in  some  degree  poisonous  through  deteriora- 
tion during  lo.ng  keeping.  The  good  effects  of  the  anti- 
scorbutics were  then  explained  as  due  to  an  antidotal 
action.  But  we  are  now  inclined  to  regard  scurvy  not 
as  a  state  of  poisoning,  but  as  a  deficiency  disease,  the  sys- 
tem being  disordered  for  want  of  some  particular  supply. 
Assuming  that  one  specific  substance  is  lacking  we  say 
there  is  need  of  a  vitamin  and  we  believe  that  it  can  be 
conveyed  in  the  various  antiscorbutics.  We  speak  some- 
what inaccurately  of  the  "  vitamin  of  scurvy,"  meaning 
the  vitamin  in  the  absence  of  which  scurvy  develops. 

Another  deficiency  disease  is  beri-beri.  This  is 
common  in  the  far  East,  while  a  condition  very  much 
like  it  has  occasionally  been  observed  in  this  country. 
It  used  to  be  thought  that  beri-beri  was  an  infectious 
disease,  and  the  impression  gained  strength  from  the 
fact  that  it  often  raged  where  men  were  closely  quar- 
tered, as  in  prisons  and  laborers'  camps.  But  these 
were  situations  in  which  the  diet  was  common  to  all 
and  not  of  a  varied  character.  Better  feeding  has 
always  stamped  out  beri-beri  and  cured  all  but  the 
most  advanced  cases. 


348  HUMAN    PHYSIOLOGY 

In  the  Orient  the  victims  of  beri-beri  have  usually  been 
people  subsisting  largely  on  rice.  The  disease  progresses 
through  two  stages.  In  the  first  there  is  steady  loss  of 
weight  and  strength.  In  the  second  there  sets  in  a 
general  neuritis,  that  is,  an  inflammation  and  de- 
generation of  the  nerve  trunks.  This  is  naturally  most 
serious  since  it  leads  to  sensory  and  motor  paralyses 
of  greater  or  less  extent.  Death  may  result  or  there 
may  be  lasting  defects  of  sensation  and  motor  control. 

Beri-beri  appears  to  be  caused  by  the  lack  of  a  vitamin. 
The  compound  is  one  which  has  apparently  been  isolated 
and  is  not  of  great  complexity.  A  neuritis  like  that  of 
beri-beri  can  be  induced  in  animals  by  restricting  them 
to  certain  foods,  and  then  overcome  by  adding  to  the 
diet  a  minute  quantity  of  the  pure  vitamin.  It  is  a 
most  interesting  fact  that  the  husk  or  pericarp  of  rice 
furnishes  the  vitamin  which  is  not  present  in  the  kernel. 
What  is  called  "polished  rice"  has  been  freed  from  the 
pericarp  and  will  cause  something  like  beri-beri  in  pigeons 
if  they  are  given  nothing  else  to  eat.  After  the  symp- 
toms are  marked  the  birds  can  be  restored  to  health 
by  including  the  husks  with  the  polished  grains. 

The  so-called  vitamin  of  beri-beri  can  be  derived  not 
only  from  the  husks  of  rice  but  from  meat,  yeast,  pota- 
toes, and  other  sources.  Any  diet  that  is  not  severely 
limited  in  variety  will  be  certain  to  afford  it.  It  can 
be  said  of  polished  rice,  as  of  gelatin,  that  it  is  not  un- 
wholesome but  merely  insufficient  by  itself.  Either 
may  be  most  valuable  when  associated  with  other  foods. 

The  course  of  events  when  beri-beri  is  coming  on  has 
been  analyzed  as  follows:  The  vitamin  is  primarily 
needed  by  the  nerves.  Some  of  it  is  present  in  other 
tissues  as  we  may  infer  from  the  remedial  virtue  of 
meat.  When  none  is  supplied  by  the  diet  the  gradual 
disintegration  of  the  vitamin  in  the  nerves  is  no  longer 
compensated  by  the  food.  The  need  will  be  met  for 
a  while  by  abstracting  the  vitamin  from  other  parts 
of  the  body.  But  to  remove  any  essential  constituent, 


THE    REQUISITES    OF   THE    DIET  349 

however  small  its  amount,  from  a  tissue  is  to  cause  the 
dissolution  of  that  tissue.  The  integrity  of  the  nerves 
is  maintained  temporarily  but  at  a  ruinous  cost.  It  is 
as  though  a  large  and  costly  machine  were  to  be  dis- 
mantled merely  to  provide  bolts  and  screws  to  repair 
another  piece  of  mechanism. 

The  period  during  which  the  tissues  in  general  are 
being  sacrificed  for  the  benefit  of  the  nerves  is  that  in 
which  the  loss  of  weight  and  strength  is  so  marked.  A 
time  arrives  when  the  internal  supply  of  the  vitamin 
as  well  as  the  supply  from  the  diet  is  insufficient  and 
the  nerves  can  no  longer  be  kept  normal.  The  stage 
of  neuritis  is  then  established. 

Other  disorders  besides  scurvy  and  beri-beri  are 
almost  certainly  to  be  classed  among  deficiency  diseases 
and  attributed  to  the  lack  of  essential  compounds  in 
the  ration.  Nutritional  difficulties  in  infancy  may  be 
examples  of  such  conditions.  There  is  at  the  present 
time  much  discussion  as  to  whether  the  serious  disease 
pellagra  belongs  in  this  class.  It  is  agreed  that  it 
attacks,  for  the  most  part,  people  whose  diet  is  monoto- 
nous and  that  liberal  feeding  favors  recovery,  but  there 
is  some  evidence  that  it  may  be  infectious.  We  cannot 
pass  judgment  here  upon  this  question. 

The  recognition  of  vitamins  gives  concreteness  to 
ideas  that  have  long  been  held  in  a  vague  way.  Many 
years  ago  Sylvester  Graham  urged  that  we  should 
not  carry  too  far  the  refining  of  food  lest,  in  the  dis- 
carded material,  we  lose  something  of  value.  The 
flour  which  bears  his  name  was  prepared  in  conformity 
with  his  teaching,  including,  as  it  does,  the  husk  as 
well  as  the  kernel.  This  was  more  than  half  a  century 
before  the  observations  concerning  the  pericarp  of 
rice. 

A  shrewd  criticism  has  been  passed  upon  sugar: 
namely,  that  it  is  not  a  normal  food  because  it  has  been 
so  refined  as  to  consist  of  only  one  compound.  Native 
food  products  are  always  mixtures  of  numerous  bodies, 


350  HUMAN    PHYSIOLOGY 

however  markedly  one  type  may  predominate.  It  does 
not  appear  that  this  argument  should  be  given  much 
weight,  for  we  use  sugar  chiefly  as  an  addition  to  other 
foods,  but  we  can  appreciate  the  suggestion  that  we 
are  not  yet  wise  enough  to  know  just  what  list  of  com- 
pounds must  be  furnished  for  all  the  purposes  of  nutri- 
tion and  which  ones  can  be  omitted.  In  default  of 
complete  information  we  ought  to  include  as  many  as 
convenient. 

Certain  agitators  contend  that  the  best  diet  is  one 
composed  entirely  of  uncooked  foods.  It  is  doubtful 
whether  a  scientific  foundation  for  such  claims  has 
ever  been  conscientiously  sought.  We  may  conceive 
that  there  are  valuable  bodies,  perhaps  deserving  to  be 
called  vitamins,  which  are  destroyed  by  heat  of  the 
degree  used  in  cooking.  In  view  of  this  possibility  it 
may  be  conceded  that  we  should  eat  a  good  deal  of  raw 
food.  But  the  advantages  of  cooking  in  developing 
flavors,  increasing  the  digestibility  of  many  articles, 
and,  above  all,  in  sterilizing  food  which  might  be  infected 
cannot  lightly  be  set  aside. 

As  it  is  possible  that  we  may  reject  needed  supplies 
by  too  much  refining  of  our  food  and  lose  something  of 
utility  in  the  processes  of  cooking,  so  we  may  possibly 
lose  vitamins  when  food  is  preserved  too  long.  When 
we  say  that  a  canned  product  has  lost  its  " goodness" 
we  are  apt  to  refer  to  flavor,  but  it  is  not  unlikely  that 
the  gradual  deterioration  that  goes  on  even  in  sterile 
food  may  rob  it  of  some  of  its  nutritive  worth.  But 
here  again  we  may  point  out  that  a  food  which  is  not 
perfectly  adapted  to  meet  all  the  current  needs  may  still 
minister  to  many  of  them  and  take  its  place  helpfully 
in  the  diet.  It  cannot  be  condemned  as  poisonous 
merely  because  it  has  its  limitations. 

It  is  evidently  much  harder  to  compare  one  diet  with 
another  than  has  been  commonly  assumed  in  the  past. 
Some  of  our  American  physiologists  have  reported  the 
excellent  results  they  have  obtained  with  rations  con- 


THE    REQUISITES    OF    THE    DIET  351 

siderably  below  the  older  standards  of  quantity.  English 
critics  have  expressed  surprise  that  the  diets  recom- 
mended have  furnished  no  more  protein  and  no  greater 
fuel-value  than  the  food  of  the  very  poor  in  London  or 
the  " punishment  ration"  of  British  prisons.  We  are 
not  disposed  to  deny  the  force  of  this  comparison,  but 
we  realize  much  more  fully  than  we  did  even  five  years 
ago  that  quality  as  well  as  quantity  must  be  taken  into 
account  in  any  such  discussion. 

We  have  seen  that  two  rations  may  be  equal  in  protein, 
as  shown  by  ordinary  analysis,  and  far  from  equal  in 
their  power  to  nourish.  If  the  protein  in  one  case  is 
from  meat  or  rice  it  will  determine  the  superiority  of 
that  diet  to.  one  in  which  the  protein  is  from  beans  or 
corn.  We  now  see,  in  addition,  that  one  ration  may 
satisfy  requirements  not  met  by  the  other  because  of 
its  minor  constituents  both  mineral  and  organic.  The 
vitamins  must  be  supposed  to  arise  as  by-products  of 
metabolism  in  the  living  matter  which  is  destroyed  to 
become  our  food;  they  cannot  be  obtained  from  proteins 
by  mere  digestion.  Quite  failing  to  modify  the  figures 
that  represent  quantitative  composition,  they  still  confer 
the  most  important  properties  upon  foods  which  contain 
them. 


CHAPTER  XXVI 
THE  HYGIENE  OF  NUTRITION 

In  the  last  chapter  it  has  been  shown  that  the  diet 
must  conform  to  certain  requirements.  It  must  furnish 
sufficient  protein  and  it  must  be  adequate  as  a  fuel 
supply.  It  has  been  shown  that  kind  as  well  as  quantity 
of  protein  must  be  considered.  Obscure  needs  must  be 
met  by  suitable  offerings  of  extractive  substances  and 
salts.  There  must  be  plenty  of  water.  It  remains  to 
speak  of  still  other  matters  related  to  individual  nutrition. 

We  need  to  remind  ourselves  that  nutritional  dis- 
turbances may  arise  from  conditions  not  directly  con- 
nected with  the  diet.  Chief  among  these  are  mental 
states.  A  brief  period  of  agitation  may  easily  lead  to 
an  indigestion.  From  what  has  been  said  of  the  nervous 
government  of  the  alimentary  tract  this  should  not 
excite  surprise.  The  failure  to  enjoy  a  meal  is  likely 
to  mean  a  failure  of  gastric  secretion  or  at  least  a  delay 
in  its  establishment.  The  motor  reactions  of  the  stomach 
may  also  be  interfered  with.  When  the  gastric  juice 
fails  to  make  its  appearance  the  food  is  decomposed  by 
bacteria  instead  of  being  digested  in  the  normal  way. 
Products  may  be  formed  which  are  sufficiently  poisonous 
to  cause  nausea,  or  the  effects  may  be  more  in  the  line 
of  gas  formation  with  pain. 

When  the  working  of  the  system  is  upset  the  stomach 
contents  may  be  retained  for  many  hours  instead  of 
being  passed  to  the  intestine.  When  they  do  reach  the 
lower  part  of  the  canal  they  are  still  subject  to  bacterial 
decomposition  and  the  products  continue  to  cause 
trouble.  If  the  normal  absorption  is  delayed,  fermenta- 
tion runs  riot  in  the  intestine  and  pain  and  flatulence 
are  to  be  expected.  If  nitrogenous  food  reaches  the 

352 


THE    HYGIENE    OF    NUTRITION  353 

colon  in  unusual  amounts,  owing  to  deficient  digestion 
and  absorption  at  higher  levels,  downright  putrefac- 
tion is  encouraged.  Poisons  are  generated  which  act 
upon  the  whole  systenvafter  being  absorbed  into  the 
blood.  Their  manifold  ill  effects  are  covered  by  the 
term  auto-intoxication. 

Some  signs  of  this  condition  have  long  been  recog- 
nized. They  include  drowsiness,  headache,  inertia, 
and  ready  susceptibility  to  fatigue.  Other  results  are 
attributed  to  the  state  when  it  is  persistent.  Among 
these  are  nervous  depression,  anemia,  troubles  with  the 
joints — popularly  called  rheumatism — and  a  disposition 
toward  hardening  of  the  arteries.  It  is  easy  to  see  that 
auto-intoxication  tends  to  perpetuate  itself.  If  it  is 
set  up  as  a  result  of  a  period  of  indigestion  it  may  pro- 
long the  causal  condition  by  depressing  the  spirits  and 
enfeebling  the  reactions  of  the  nervous  system.  Here 
is  a  good  example  of  an  effect  becoming  a  cause  and  so 
acting  as  to  intensify  itself.  This  is  what  is  known  as  a 
"vicious  cycle." 

The  system  will  generally  rally  from  indigestion  that 
has  its  origin  in  transient  unhappiness,  anger,  or  pain. 
When  such  moods  are  long  continued  as  in  grief,  anxiety, 
severe  disappointment,  or  intense  homesickness  auto- 
intoxication may  become  a  fixed  condition  that  will  not 
remedy  itself  with  the  passing  of  the  circumstances  that 
were  primarily  responsible.  The  bacterial  activities 
that  are  rife  in  the  intestine  may  require  something  quite 
different  from  mental  suggestion  for  their  regulation. 

We  cannot  avoid  intestinal  infections.  The  canal 
is  sterile  at  birth,  but  never  remains  so  for  more  than  a 
few  hours.  Within  a  few  weeks  it  harbors  a  countless 
host  of  microorganisms  of  many  varieties,  subsisting 
upon  the  food  and  secretions,  multiplying,  and  perish- 
ing to  undergo  digestion  like  other  organic  matter. 
Billions  or  trillions  of  them,  living  and  dead,  are  thrust 
out  with  the  feces  and  yet  the  supply  is  maintained. 
It  is  not  certain  that  man  derives  any  advantage  from 

23 


354  HUMAN    PHYSIOLOGY 

the  presence  of  the  intestinal  bacteria  but,  on  the  other 
hand,  in  the  majority  of  cases  they  are  not  known  to 
harm  him.  Mischief  may  be  done  either  by  a  general 
excess  of  bacterial  activity  or  by  the  substitution  of 
hurtful  for  innocuous  forms. 

Measures  against  Auto-intoxication. — We  have  sug- 
gested that  this  undesirable  condition  may  result  when 
the  digestion  has  been  retarded  as  a  r.esult  of  psychic 
factors.  It  may  equally  well  follow  disturbances  set 
up  in  other  ways.  Overeating  is  probably  a  frequent 
cause.  If  the  capacity  of  the  canal  to  absorb  all  the 
food  presented  is  overtaxed  there  will  be  residues  to  be 
decomposed.  It  is  fortunate  that  in  many  cases  of 
overindulgence  a  mild  diarrhea  is  the  most  obvious 
sequel.  Auto-intoxication  is  minimized  by  the  vigorous 
sweeping  out  of  the  threatening  material.  A  cathartic 
may  insure  the  same  protection,  but  should  not  be 
resorted  to  at  all  frequently. 

It  might  be  supposed  that  overeating  would  lead 
automatically  to  loss  of  appetite  and  so  correct  itself. 
But  we  cannot  rely  upon  any  such  adjustment.  A 
slightly  excessive  consumption  of  food  may  have  no 
evident  injurious  effect  for  long  periods  of  time,  yet  the 
ultimate  result  may  be  the  shortening  of  active  life 
through  the  early  development  of  arteriosclerosis. 
It  almost  seems  as  though  the  subtle  changes  induced 
by  such  a  small  dietetic  error  were  more  serious  than 
those  likely  to  follow  a  more  gross  transgression. 

Temperance  in  eating  is  doubtless  the  chief  practice 
to  be  recommended  as  a  safeguard  against  auto-intoxica- 
tion and  its  evils.  Temperance  in  drinking  might  be 
mentioned  at  the  same  time,  for  the  moderate  drinker 
is  disposed  to  overeat  and  alcohol  itself  is  a  common 
cause  of  arteriosclerosis.  It  is  more  important  to 
restrict  the  protein  food  than  the  other  kinds,  for  the 
definitely  poisonous  compounds  entering  the  blood 
from  the  colon  are  believed  to  be  nitrogenous.  Modera- 
tion in  meat-eating  is  to  be  urged,  but  the  reason,  it 


THE    HYGIENE    OF    NUTRITION  355 

should  be  pointed  out,  lies  in  the  peculiar  attractive- 
ness of  meat  rather  than  in  its  composition.  Eggs, 
beans,  and  peas  might  have  as  much  to  do  with  auto- 
intoxication if  they  were  as  appealing  to  the  average 
appetite. 

It  will  be  noted  that  we  can  avoid  toxic  decomposition 
in  the  intestine  either  by  keeping  the  tract  as  free  as 
possible  from  lagging  residues  or  by  controlling  the 
prevalent  type  of  bacterial  action.  This  latter  pro- 
cedure has  had  many  advocates.  It  is  taught  that  the 
more  harmful  microorganisms  can  be  kept  from  undue 
increase  if  less  objectionable  species  are  deliberately 
encouraged.  This  is  the  theory  of  the  various  sour-milk 
treatments.  The  familiar  change  that  takes  place  in 
milk  is  a  fermentation  of  milk-sugar  with  the  formation 
of  lactic  acid.  Although  lactic  acid  in  the  muscular 
and  nervous  tissues  of  the  body  is  clearly  undesirable, 
it  seems  that  a  good  deal  of  it  may  be  introduced  into 
the  alimentary  canal  or  produced  there  without  ill 
effects.  Certain  kinds  of  bacteria  which  form  lactic 
acid  from  sugars  at  a  rapid  rate  can  be  isolated,  grown 
in  quantity,  and  dispensed. 

The  subject  may  prepare  sour  milk  by  means  of  these 
cultures  and  drink  it,  swallowing  the  acid  and  the  pro- 
ducing organisms  at  once.  Or  he  may  prefer  to  swallow 
the  cultures  together  with  some  sugar  which  may  then 
be  fermented  within  the  tract.  In  proportion  as  this 
type  of  fermentation  becomes  the  dominant  one  certain 
other  decompositions  are  inhibited.  It  is  a  curious  fact 
that  something  can  be  learned  about  the  state  of  the 
colon  by  studying  the  urine.  There  are  well-recognized 
products  of  protein  putrefaction  that  may  be  absorbed 
into  the  circulation  and  later  excreted  in  modified  form 
by  the  kidneys.  The  amount  of  these  compounds  in 
the  urine  is  thus  an  index  of  the  extent  of  objectionable 
decomposition  in  the  lower  part  of  the  canal.  Lactic 
acid  treatments  often  reduce  the  quantity  of  these  tell- 
tale substances. 


356  HUMAN    PHYSIOLOGY 

It  has  been  suggested  in  Chapter  XV  that  the  human 
colon  is,  perhaps,  as  much  a  menace  as  a  support  to  the 
individual.  The  idea  that  man  would  be  better  with- 
out it  has  been  freely  entertained.  When  it  has  been 
absolutely  necessary  to  remove  it  and  the  immediate 
dangers  of  the  operation  have  been  passed,  there  has 
been  no  trouble  in  maintaining  good  nutrition.  The 
discharges  are  watery  and  voluminous  but  contain 
little  more  than  the  normal  food  residues.  In  a  few 
cases  the  colon  has  been  removed,  not  because  of  local 
disease,  but  in  the  hope  of  bettering  the  general  health 
through  checking  auto-intoxication.  Occasionally  the 
measure  seems  to  be  justified  by  the  happy  results  ob- 
tained but  surgeons  do  not  recommend  it  unless  the 
condition  is  quite  grave. 

Und creating. — Most  of  the  popular  writing  on  topics 
of  this  kind  presses  home  the  teaching  that  most  people 
eat  more  than  they  should.  There  can  be  no  doubt 
that  overeating  is  common,  especially  among  men,  and 
most  of  all  among  those  who  have  plenty  of  money. 
But  we  can  hardly  question  that  there  are  multitudes 
of  people  who  are  underfed.  There  are  the  very  poor 
and  there  are  those  with  more  adequate  incomes  who 
still  economize  unwisely  upon  their  food.  Women  are 
prone  to  make  this  mistake.  They  prefer  to  have  money 
for  so  many  other  things  that  they  do  not  allow  them- 
selves enough  to  eat.  Often  they  are  alone  in  their 
homes  at  noon  and  are  too  indifferent  to  make  the 
exertion  of  getting  a  square  meal.  Appetite  soon  flags 
and  a  habit  of  undereating  is  established. 

People  who  do  not  eat  enough  are  likely  to  be  under- 
weight, pale,  and  sensitive  to  cold.  They  are  often 
good  workers  but  in  their  performance  they  are  apt  to 
appear  conscientious  rather  than  enthusiastic.  They 
give  an  impression  of  having  little  energy  to  spare.  They 
are  apt  to  be  light  and  fitful  sleepers  if  not  actual  sufferers 
from  insomnia.  But  the  most  evident  effects  of  under- 
feeding are  generally  observed  in  the  mental  attitude. 


THE    HYGIENE    OF    NUTRITION  357 

These  persons  are  depressed  or  morose,  often  querulous 
and  prejudiced,  seeming  to  be  their  own  worst  enemies. 
The  contrast  which  they  present  with  the  overfed  is  a 
consistent  one.  The  man  who  eats  too  much — and 
escapes  indigestion — is  usually  overweight,  florid,  not 
very  diligent,  a  heavy  sleeper,  and  an  optimist.  There 
are  drawbacks  in  either  case.  The  picture  of  the 
underfed  subject  is  much  the  same  whether  his  habit 
is  the  result  of  outward  circumstances  or  alimentary 
incapacity. 

Food  Poisoning. — We  must  indicate  a  distinction  be- 
tween auto-intoxication  which  is  due  to  the  production 
of  poisons  in  the  canal  and  the  acute  attacks  which 
may  be  occasioned  by  food  that  has  become  poisonous 
before  it  is  eaten.  Such  changes  in  food  are  fortunately 
rare.  We  need  to  reflect  that  very  extensive  decom- 
position may  occur  without  making  food  dangerous. 
The  supreme  examples  are  furnished  by  certain  cheeses 
which  have  been  so  treated  as  to  advance  putrefaction 
to  the  utmost.  They  rarely  cause  sickness. 

From  time  to  time  we  hear  of  outbreaks  described  as 
ptomain  poisoning.  They  are  undoubtedly  less  frequent 
than  they  were  thirty  years  ago,  for  greater  intelligence 
in  regard  to  the  control  and  inspection  of  food  has  had 
a  favorable  effect.  A  ptomain  is  defined  as  a  bacterial 
product  derived  from  nitrogenous  food.  It  is  usually 
assumed  to  be  rather  simple  in  its  chemical  nature  and 
has  been  described  as  an  animal  alkaloid.  This  is  not 
a  good  term  for  we  can  probably  obtain  the  same 
ptomains  from  vegetable  proteins  as  from  those  of  meat. 
The  word  ptomain  need  not  of  necessity  suggest  a  poison, 
but  it  is  usually  so  understood  for,  among  many  com- 
pounds released  simultaneously  in  certain  types  of  de- 
composition, there  are  some  which  are  intensely  toxic. 

Meat  and  fish,  including  shellfish,  have  figured  in 
most  of  the  spectacular  cases.  A  number  of  persons 
who  have  eaten  food  from  a  common  source,  it  may  be 
at  a  banquet,  have  become  violently  ill.  A  few  hours 


358  HUMAN    PHYSIOLOGY 

after  the  meal  they  are  seized  with  agonizing  pangs, 
uncontrollable  diarrhea,  and,  in  most  instances,  forcible 
vomiting.  The  temperature  rises  to  a  fever  pitch,  there 
is  extreme  prostration,  and  death  from  exhaustion  may 
occur.  But  the  reaction  of  the  system  evidently  favors 
a  thorough  removal  of  the  poison  and  recovery  is  usually 
prompt. 

There  are  odd  cases  of  food  poisoning  which  present 
an  entirely  different  group  of  symptoms :  a  painless  pros- 
tration, with  more  or  less  paralysis,  and  stupor.  The 
poisons  which  produce  such  effects  must  be  of  a  narcotic 
nature.  The  outlook  is  worse  under  these  conditions 
than  in  the  ordinary  disturbance  for  it  is  most  difficult 
to  clear  the  sluggish  and  benumbed  system  of  the  agent 
that  is  threatening  its  life.  Poisoning  by  decomposed 
mussels,  molluscs  eaten  in  Germany,  has  often  had  this 
narcotic  character,  and  so  has  the  sausage  poisoning 
which  has  been  recorded  in  the  same  country. 

There  is  no  apparent  reason  why  other  nitrogenous 
foods  than  meat  should  not  undergo  poisonous  decom- 
position. In  fact  we  have  reports  of  sickness  due  to 
string  beans  which  had  been  imperfectly  preserved  in 
glass.  There  is  widespread  objection  to  canned  goods, 
but  it  does  not  appear  that  they  have  been  responsible 
for  much  acute  poisoning.  Neither  does  it  seem  that 
much  damage  has  been  done  by  metals  and  preserva- 
tives. Ice  cream  sometimes  becomes  excessively  poison- 
ous; it  may  be  vomited  almost  instantly.  Those  who 
have  studied  poisonous  ice  cream  most  carefully  do  not 
believe  that  the  metal  from  the  freezer  is  concerned,  but 
rather  that  it  is  a  true  bacterial  alteration  of  the  milk. 

In  times  past,  more  or  less  illness  has  been  caused 
by  the  consumption  of  meat  from  diseased  animals. 
Unscrupulous  men  have  hastily  slaughtered  and  marketed 
animals  which  were  about  to  die.  (One  is  reminded  of 
the  permission  conveyed  in  Deuteronomy,  xiv,  21.) 
Such  criminal  acts  are  more  effectively  guarded  against 
in  these  days.  When  we  consider  the  possibility  of 


THE    HYGIENE    OF    NUTRITION  359 

damage  being  done  by  such  meat  we  see  that  two  dis- 
tinct results  may  follow:  the  same  disease  that  the 
animal  had  may  be  transmitted  to  man,  or  there  may  be 
a  poisoning  from  the  abnormal  state  of  the  tissues. 
Thorough  cooking  will  protect  against  the  infection, 
but  it  is  not  at  all  certain  to  neutralize  the  poisonous 
properties  of  the  flesh. 

We  hear  it  said  that  "one  man's  meat  is  another 
man's  poison."  This  is  rather  an  extreme  statement  as 
related  to  the  cases  it  is  usually  intended  to  cover,  but 
it  may  be  literally  true.  There  are  the  most  curious 
idiosyncrasies  toward  particular  foods  on  the  part  of 
individuals.  Some  cannot  eat  eggs,  others  are  made 
sick  by  lobster  or  crab  meat.  Certain  persons  are 
poisoned  seriously  by  potato.  Strawberries  cause  skin 
eruptions  in  many  subjects.  It  has  been  surmised  that 
some  of  these  unfortunate  reactions  are  due  to  suggestion, 
the  painful  memory  of  a  past  illness  making  a  fresh  trial 
with  normal  confidence  out  of  the  question.  It  is  certain 
that  some  are  of  a  more  fundamental  sort,  the  food  pro- 
ducing its  effect  however  perfectly  it  has  been  disguised. 

Constipation. — Much  is  written  of  this  condition. 
A  great  part  of  that  which  comes  before  the  average 
reader  is  designed  to  promote  the  sale  of  cathartics. 
It  has  to  be  discounted  accordingly,  and  yet  there  is  no 
doubt  that  constipation  does  much  harm.  Those  who 
are  most  nearly  immune  to  evil  consequences  are  the 
small  eaters.  The  "  Fletcherite "  who  practises  pro- 
longed mastication  and  subsists  on  the  lowest  possible 
ration  may  have  only  one  or  two  evacuations  a  week 
and  still  feel  well  and  make  a  creditable  showing  when 
tested  for  mental  and  muscular  capacity.  He  is  saved 
from  auto-intoxication  by  the  small  amount  and  dry 
character  of  the  intestinal  content. 

The  more  liberal  feeder  is  safer  if  he  adheres  to  the 
time-honored  rule  of  one  movement  a  day.  Regularity 
is  not  an  absolute  disproof  of  a  constipated  condition 
for  there  may  be  an  undesirable  lag  in  the  progress  of 


360  HUMAN    PHYSIOLOGY 

material  along  the  canal.  Not  only  should  there  be  a 
daily  unloading  of  the  colon,  but  the  feces  should  corre- 
spond with  the  intake  of  the  previous  day  rather  than 
of  some  day  farther  back.  There  is  probably  a  great 
difference  between  the  maximum  and  the  minimum  rate 
of  travel  along  the  individual  intestines  of  a  group  of 
people  who  all  consider  themselves  free  from  constipa- 
tion. The  slower  the  rate  of  advance  the  larger  the 
amount  of  matter  present  at  a  given  time  to  give  rise 
to  poisons. 

It  may  even  happen  that  a  state  of  constipation  shall 
have  symptoms  of  diarrhea.  There  may  be  heavy  ac- 
cumulations in  the  colon  which  its  contractions  are  too 
weak  to  displace  and  a  catarrhal  discharge  from  the  irri- 
tated regions  may  create  an  entirely  false  impression. 
The  advice  of  a  good  physician  may  be  needed  to  de- 
termine what  course  shall  be  pursued. 

The  old-time  doctor  gave  a  powerful  cathartic  on 
almost  every  occasion.  Many  of  the  slight,  nameless 
illnesses  we  suffer,  especially  in  childhood,  yield  imme- 
diately to  this  measure.  The  quick  return  of  normal 
feeling  seems  to  favor  the  view  that  the  trouble  was 
an  auto-intoxication  and  that  the  toxic  matter  has 
been  adequately  removed,  but  it  is  the  part  of  wisdom 
to  limit  the  number  of  occasions  for  resorting  to 
such  means  of  relief.  Most  people  can  avoid  recur- 
rences by  taking  reasonable  exercise,  drinking  a  good 
deal  of  water,  and  eating  fruit  and  coarse  vegetables. 

Washing  out  the  colon  now  and  then  is  a  procedure 
which  has  its  devotees.  It  has  a  real  value  in  special 
cases,  but  is  to  be  avoided  unless  prescribed  by  the 
physician.  A  dependence  upon  the  enema  may  be 
established  which  is  in  the  highest  degree  irksome  and 
little  better  in  principle  than  a  cathartic  habit.  It  may 
be  said  incidentally  that  one  marked  difference  between 
the  reaction  to  a  purgative  and  an  enema  lies  in  the 
fact  that  the  drug  usually  contracts  the  output  of  the 
kidneys  by  diverting  water  to  the  intestine,  while  the 


THE    HYGIENE    OF    NUTRITION  361 

enema  encourages  the  absorption  of  water  and  increases 
the  volume  of  the  urine.  Irrigation  of  the  colon  is 
accordingly  helpful  when  it  is  desired  to  promote 
elimination  by  the  kidneys,  as  in  some  cases  classed  as 
rheumatism  and  arthritis. 

Obesity. — Increase  of  adipose  tissue  can  occur  only 
when  the  income  of  the  body  is  for  some  time  in  excess 
of  the  fuel  requirement.  This  is  a  plain  fact,  but  one 
which  is  not  always  recognized.  It  would  seem  to 
follow  that  such  increases  could  be  counteracted  either 
by  restricting  the  diet  or  by  speeding  up  the  oxidation. 
Both  objects  are  often  sought.  If  the  food  is  limited 
the  weight  must  diminish.  When  we  are  not  eating  or 
drinking  we  are  necessarily  losing  weight  at  the  rate  of 
at  least  an  ounce  an  hour.  But  fasting  is  not  pleasant 
and  insufficient  feeding  may  be  even  more  distressing. 

The  consumption  of  adipose  tissue  may  be  forced  by 
exercise.  A  common  difficulty  is  that  the  appetite  is 
so  stimulated  that  unless  it  is  sternly  curbed  the  accumu- 
lation is  at  once  replaced.  The  victim  has  his  labor  for 
his  pains.  We  have  made  a  brief  reference  to  the  power 
of  thyroid  extracts  to  accelerate  the  metabolism  and  to 
the  use  of  these  preparations  in  medicines  for  weight 
reduction.  They  are  not  ideal  for  the  purpose  since 
they  produce  nervousness  and  disturbances  of  the  heart 
action. 

The  choice  of  a  diet  containing  an  unusual  quantity 
of  protein  has  been  recommended  for  the  purpose  of 
cutting  down  the  weight.  The  old  theory  was  that 
protein  could  make  muscle  but  not  fat,  an  idea  favored 
by  the  leanness  of  carnivorous  animals.  We  have  been 
brought  to  believe  that  fat  can  be  made  from  protein, 
with  sugar  as  an  intermediate  stage.  The  effect  of  a 
high  protein  diet  can  be  explained  as  due  to  two  factors. 
In  the  first  place,  it  is  likely  to  be  satiating  and  the 
amount  eaten  is  automatically  reduced  without  any 
suffering  from  hunger  and  faintness.  In  the  second 
place,  protein,  more  than  other  types  of  food,  stimulates 


362  HUMAN    PHYSIOLOGY 

the  rate  of  oxidation.  The  technical  expression  is 
that  "it  has  a  marked  specific  dynamic  effect." 

One  system  of  treating  obesity  depends  on  the  rather 
heroic  principle  of  giving  drugs  which  keep  the  patient 
in  a  qualmish  condition  and  abolish  his  appetite.  Fast- 
ing is  thus  made  easy,  but  it  does  not  seem  as  though  the 
after-effect  upon  the  organs  of  digestion  could  be  good. 
One  might  choose  to  be  fat  rather  than  dyspeptic. 

The  great  trouble  with  most  of  the  methods  employed 
to  reduce  weight  is  that  the  constitutional  tendency  is 
unaltered  by  them  and  results  are  apt  to  be  temporary. 
The  subject  faithfully  follows  a  routine  at  the  cost  of 
much  self-sacrifice  and  rapidly  regains  the  loathed  adi- 
pose tissue  when  he  changes  his  mode  of  life.  Perhaps 
the  most  practical  suggestion  for  a  line  of  conduct  that 
can  be  kept  up  indefinitely  is  that  bulk  rather  than 
nutriment  be  sought  after.  The  clamors  of  the  stomach 
can  be  stilled  by  filling  it  with  fruit,  green  vegetables, 
pop  corn,  etc.,  instead  of  with  bread  and  butter,  potato, 
pastry,  and  candy.  It  is  a  policy  of  self-deception  but 
warranted  in  a  good  cause. 

The  Teeth. — Digestion  and  nutrition  depend  to  a 
considerable  degree  upon  mastication.  It  is  probable 
that  too  much  virtue  has  been  claimed  for  the  chewing 
of  the  food,  but  it  is  certainly  better  to  err  in  the  direc- 
tion of  excessive  rumination  than  to  become  careless  in 
regard  to  it.  The  teeth  are  to  be  conserved  and  used. 
It  might  be  thought  that  the  vigorous  employment  of 
the  teeth  could  only  hasten  their  wear  and  tear.  This 
is  probably  the  case  at  a  time  when  their  life  is  extinct 
or  limited  to  a  small  central  core,  but  at  an  earlier  period 
mastication  appears  to  be  good  for  the  teeth.  This  is 
because  they  are  made  to  sink  and  rise  in  their  sockets 
with  a  massaging  effect  upon  the  gums  and  some  pro- 
motion of  the  circulation  in  the  pulps. 

According  to  the  usual  teaching  the  best  protection 
to  the  teeth  is  afforded  by  the  use  of  an  alkaline  mouth 
wash  such  as  milk  of  magnesia.  If  this  is  used  at  bed- 


THE    HYGIENE    OF    NUTRITION  363 

time  it  should  not  be  rinsed  out  but  left  in  the  by-places 
of  the  mouth  to  guard  against  the  development  of  an 
acid  reaction.  The  brushing  of  the  teeth  may  be  over- 
done; it  should  cleanse  their  surfaces  but  should  not 
be  so  directed  as  to  encourage  recession  of  the  gums. 
It  is  often  assumed  that  people  with  poor  teeth  are  pay- 
ing the  penalty  for  their  neglect.  Sometimes,  of  course, 
this  is  a  fact,  but  it  is  also  true  that  many  people  have 
superb  teeth  which  they  owe  entirely  to  good  fortune 
and  not  to  conscientious  care. 

The  assistance  of  the  dentist  must  be  had  at  short 
intervals  by  many  subjects.  Teeth  which  are  in  need 
of  filling  or  other  treatment  are  most  detrimental.  They 
are  not  merely  disfiguring  and  responsible  for  bad  breath, 
but  their  presence  deters  the  possessor  from  using  proper 
force  in  mastication.  It  is  believed  in  addition  that 
defective  teeth  are  often  foci  from  which  most  injurious 
infections  are  spread  to  other  parts  of  the  system.  The 
ugly  fact  stands  out  that  our  teeth,  being  incapable  of 
self-repair,  force  upon  us  an  early  reminder  of  our 
mortality. 


CHAPTER  XXVII 
THE  MAINTENANCE  OF  THE  BODY  TEMPERATURE 

One  of  the  most  impressive  examples  of  coordination 
is  afforded  by  the  associated  mechanisms  which  so  suc- 
cessfully preserve  the  human  body  temperature  from 
violation.  Summer  and  winter,  indoors  and  out,  in 
the  polar  regions  and  in  the  Sahara,  it  seems  independent 
of  external  conditions.  A  discussion  of  the  facts  has 
been  postponed  to  this  time  because  almost  all  the 
other  divisions  of  our  science  are  needed  as  a  founda- 
tion. The  manner  of  working  of  the  nervous  system 
with  its  receptors  and  effectors,  the  nature  of  the  con- 
tractile process  in  skeletal  muscle,  the  government  of 
the  circulation,  and  the  factors  in  metabolism  must  all 
be  in  mind. 

Most  animals  do  not  have  this  wonderful  ability  to 
hold  themselves  to  a  constant  temperature.  Birds 
and  mammals  generally  possess  the  power  and  we  indi- 
cate it  when  we-  call  them  warm-blooded.  Speaking 
accurately,  we  do  not  mean  so  much  to  emphasize  their 
warmth  as  the  constancy  of  their  internal  state.  If  a 
fish  and  a  duck  are  both  swimming  in  water  having  a 
temperature  of  103°F.,  the  two  may  have  the  same  in- 
ternal temperature.  But  if  the  water  cools  down  the 
fish  will  passively  submit  to  a  parallel  cooling  while  the 
duck  will  be  about  as  warm  as  before,  so  far  as  its  deeper 
tissues  are  concerned.  When  we  call  the  fish  a  cold- 
blooded animal  we  mean  that  it  takes  very  nearly  the 
temperature  of  its  surroundings. 

In  the  interest  of  precision  we  may  substitute  for 
warm-blooded  the  word  homothermous,  which  means  hav- 
ing a  uniform  temperature.  The  standards  so  strictly 

364 


MAINTENANCE    OF    THE   BODY    TEMPERATURE      365 

adhered  to  by  birds  and  mammals  are  in  the  same 
general  region,  say  between  97°  and  110°.  The  highest 
temperatures  are  those  of  small  birds.  It  was  suggested 
in  Chapter  XVI  that  the  salts  of  the  plasma  are  rem- 
iniscent of  those  in  the  pre-historic  sea  from  which  ances- 
tral forms  of  life  came  to  land.  It  may  also  be  suggested 
that  the  temperatures  so  faithfully  perpetuated  by  birds 
and  mammals  of  the  present  are  those  which  were  im- 
pressed upon  the  early  organisms  by  the  water  in  which 
they  lived.  It  may  be  argued  that  if  this  is  so  the  cold- 
blooded animals  are  really  the  ones  which  have  more  per- 
fectly met  the  geological  conditions  of  our  time.  The 
failure  of  the  homothermous  to  adjust  themselves  obliges 
them  to  metabolize  a  great  deal  more  food  than  would 
be  necessary  if  they  took  the  temperatures  of  their 
environment.  The  low  temperature  of  fishes  does  not 
prevent  them  from  being  very  active ;  on  the  other  hand, 
we  do  not  find  much  cerebral  capacity  in  any  of  the 
cold-blooded  species. 

When  we  speak  of  constant  temperatures  we  have 
always  in  mind  the  internal  and  not  the  surface  condi- 
tions. The  temperature  of  the  human  skin  cannot  be 
constant  since  it  is  influenced  by  the  varying  state  of 
the  air  as  well  as  by  the  changes  in  the  circulation. 
The  skin  temperature  approaches  that  of  the  blood 
most  nearly  when  the  vessels  are  locally  dilated  and  when 
a  covering  retards  the  loss  of  heat  to  the  air.  We 
know  that  an  exposed  spot  is  not  proof  against  frost- 
bite which  shows  how  the  regulating  mechanism  may 
be  overmatched. 

The  clinical  thermometer  may  be  used  in  the  mouth, 
the  armpit,  or  the  rectum.  Occasionally  other  localities 
are  utilized  or  the  temperature  of  the  urine  may  be  taken. 
If  the  mouth  and  rectal  temperatures  are  taken  at  the 
same  time  the  latter  will  generally  be  the  higher  by  about 
half  a  degree.  The  difference  is  increased  when  there  is 
deep  breathing  because  of  the  cooling  effect  on  the 
whole  region  about  the  mouth.  Runners  who  are 


366  HUMAN    PHYSIOLOGY 

panting  from  long  exertion  may  show  an  elevation  of 
the  rectal  temperature  but  no  corresponding  rise  in  the 
mouth. 

There  is  a  slight  daily  rise  and  fall  of  the  temperature 
wherever  measured.  The  lowest  point  is  passed  in 
the  early  morning  before  the  reluctant  muscles  have 
been  roused  to  much  activity.  There  is  an  upward 
tendency  through  the  day  and  the  highest  level  is  reached 
late  in  the  afternoon  or  early  in  the  evening.  The 
extent  of  the  fluctuation  is  rather  more  than  1°F.  When 
there  is  fever  the  same  ascending  curve  is  usually  re- 
corded and  the  patient  is  likely  to  be  more  restless  and 
uncomfortable  toward  night. 

While  we  recognize  the  daily  variation  it  is  still  plain 
that  the  approximate  constancy  of  the  body  tempera- 
ture is  the  outstanding  fact.  Where  heat-production  is 
always  going  on  uniformity  of  temperature  must  be 
due  to  a  balance  between  this  production  and  the  con- 
current loss,  between  thermogenesis  and  thermotaxis,  if 
we  adopt  the  language  of  science.  The  place  and 
manner  of  heat-production  are  already  familiar;  it  occurs 
chiefly  in  the  skeletal  muscles  with  supplementary  con- 
tributions from  the  heart  and  the  glands.  We  have 
now  to  add  some  details  regarding  heart  loss. 

The  total  for  the  day  is  that  of  the  metabolism;  we 
will  again  assume  2500  Calories.  We  know  that  some 
of  this  heat  is  imparted  to  the  air  and  the  surroundings 
in  general.  Another  fraction  of  it  becomes  latent 
through  the  evaporation  of  water.  The  relative  amount 
of  the  two  portions  will  depend  on  the  external  tem- 
perature. If  this  is  low,  as  in  winter,  the  subject  will 
evaporate  much  less  water  than  if  it  is  high.  A  water 
loss  from  the  breathing  passages  and  the  skin  aggre- 
gating 1000  grams  will  render  latent  over  500  Calories, 
as  previously  stated.  The  remaining  Calories  of  our 
assumed  total — about  2000 — will,  in  this  case,  be  directly 
measurable  as  heat  given  to  the  surroundings. 

A  proportion  like  that  just  suggested   may  be  ex- 


MAINTENANCE    OF    THE    BODY    TEMPERATURE       367 

pected  with  a  moderate  external  temperature.  If  the 
chamber  is  made  warmer  the  share  represented  by 
evaporation  will  be  progressively  increased  until,  with 
an  outside  temperature  equal  to  that  of  the  body,  all 
the  heat  produced  must  be  made  latent  by  evaporation. 
To  get  rid  of  the  whole  2500  Calories  in  twenty-four 
hours,  a  man  in  a  chamber  at  99°F.  would  be  compelled 
to  secrete  through  the  respiratory  tract  and  the  sweat 
glands  nearly  5  liters  (more  than  a  gallon)  of  water. 
He  could  not  add  measurable  heat  to  his  environment — 
that  is,  he  could  not  make  it  warmer — when  it  was  at 
his  own  temperature  in  the  first  place. 

A  man  subjected  to  this  severe  trial  would  not  give 
off  extra  water  to  any  extent  by  increasing  his  breath- 
ing, but  almost  wholly  by  increasing  his  secretion  of 
sweat.  Animals  like  the  dog  which  pant  when  they 
are  in  a  warm  atmosphere  rid  themselves  of  surplus 
heat  almost  entirely  by  increasing  evaporation  from 
the  mouth  and  throat  and  the  extended  tongue.  We 
are  not  to  assume  that  their  behavior  indicates  any  such 
distress  as  it  would  in  the  human  subject. 

As  the  skin  is  warmed  the  receptors  which  lie  just 
under  its  surface  are  stimulated.  The  reflexes  which 
result  are  both  vasomotor  and  secretory.  The  vessels 
of  the  skin  are  dilated  and  a  larger  share  of  the  total 
blood-stream  than  usual  finds  its  way  through  them. 
This  favors  the  cooling  of  the  blood  unless  the  external 
temperature  is  equal  to  or  above  the  internal.  With 
the  ordinary  balance  reversed  the  cutaneous  dilation 
would  cease  to  be  a  protective  reaction  if  it  did  not 
help  to  sustain  the  sweat  glands  in  their  augmented 
activity. 

Humidity. — The  possibility  of  evaporating  water  de- 
pends on  the  extent  to  which  the  air  is  already  satu- 
rated— in  other  words,  on  the  humidity.  If  the  air  in 
contact  with  the  skin  holds  all  the  water  which  it  can, 
we  cannot  expect  it  to  take  more  save  under  one  im- 
portant condition:  it  may  be  warmed  and  gain  in  its 


368  HUMAN    PHYSIOLOGY 

capacity  to  contain  water  vapor.  We  have  said  that 
this  happens  when  air  fully  saturated,  but  cooler  than 
the  tissues,  is  respired.  It  happens  also  when  such  air 
blows  over  the  surface  of  the  body.  High  humidity  is, 
accordingly,  not  a  serious  obstacle  to  evaporation  from 
the  skin  unless  the  air  is  exceptionally  warm. 

The  coincidence  of  high  temperature  with  high 
humidity  is  uncomfortable  and  even  dangerous.  We 
know  that  the  most  trying  days  of  the  summer  are  not, 
as  a  rule,  the  ones  on  which  the  highest  records  are 
made  by  the  thermometer;  they  are  rather  the  days 
which  we  characterize  as  sticky  and  lifeless.  Loss  of 
heat  from  the  body  is  impeded  by  the  limitation  of 
evaporation  as  well  as  by  the  actual  warmth  of  the  sur- 
roundings. It  has  been  found  that  men  working  in 
deep  mines  where  it  is  both  warm  and  moist  may  have 
continuous  fever  temperatures.  Their  situation  is  almost 
intolerable.  The  miner  may  be  in  a  much  worse  position 
than  the  stoker  of  the  steamer  who  endures  a  much 
hotter  atmosphere. 

The  air  that  is  brought  into  the  fireroom  of  the  steamer 
is  fresh  from  the  cool  exterior  and  when  it  is  heated  it 
comes  to  have  a  very  low  relative  humidity.  It  is  well 
adapted  to  take  up  the  water  which  is  secreted  in  such 
abundance  by  the  toilers.  We  must  call  attention  to  the 
fact  that  we  cannot  judge  the  amount  of  the  perspiration 
by  the  appearance  of  the  skin.  So  long  as  evaporation 
fully  keeps  pace  with  the  production  there  may  be  no 
visible  moisture.  It  is  when  evaporation  lags  behind  the 
outpouring  that  we  notice  the  drops.  We  are  actually 
most  conscious  of  the  perspiration  when  it  is  failing  to 
accomplish  its  purpose.  An  English  student  of  these 
problems  has  asserted  that  a  man  cannot  work  with 
safety  in  fully  saturated  air  at  90°F.,  but  can  be  pro- 
tected by  warming  the  same  air  by  40°.  The  paradox  is 
easily  explained.  Heating  the  air  gives  it  a  low  relative 
humidity  and  the  evaporation  which  was  very  slight  at 
90°  becomes  extremely  rapid  at  130°. 


MAINTENANCE    OF    THE    BODY    TEMPERATURE       369 

Individuals  have  endured  incredibly  high  temperatures 
when  the  conditions  have  been  favorable  to  free  evapora- 
tion. In  the  eighteenth  century  a  series  of  trials  was 
made  by  three  members  of  the  Royal  Society  of  London 
and  they  finally  achieved  the  distinction  of  having 
remained  for  a  quarter  of  an  hour  in  an  atmosphere  at 
225°.  Water  boils  at  212°.  Their  feet  were  protected  by 
thick  straw  sandals  and  it  is  recorded  that  one  of  them 
blistered  his  hand  by  touching  his  watch  chain.  A  piece 
of  meat  was  cooked  during  the  experiment  by  simply 
exposing  it  on  a  plate.  The  survival  of  these  men  and 
others  in  similar  cases  must  be  due  to  the  presence  close 
to  all  the  surfaces  of  the  body,  including  the  respiratory 
tract,  of  a  layer  of  relatively  cool  air. 

Radiation  and  Conduction. — Heat  which  escapes  from 
the  body  to  warm  the  surroundings  is  transmitted  in  two 
ways.  A  part  of  it  is  conducted.  By  this  we  mean  that 
it  raises  the  temperature  of  matter  in  direct  contact 
with  the  body  and  then  that  of  more  matter  just  beyond 
the  first.  Heat  is  conducted  from  a  man's  body  to  the 
bricks  of  the  cold  wall  against  which  he  leans.  It  is 
conducted  to  the  cold  water  through  which  he  swims. 
Some  substances  are  superior  conductors  of  heat  and 
these  feel  cooler  than  others  of  the  same  temperature. 
Iron  feels  colder  than  wood  because  it  receives  heat  from 
the  skin  at  a  more  rapid  rate,  the  temperatures  of  the  iron 
and  the  wood  being  equal. 

By  radiation  we  mean  the  very  swift  departure  of  heat 
through  space.  We  radiate  heat  through  the  air  without 
heating  the  air  itself  to  any  appreciable  extent.  Heat 
may  pass  from  the  human  body  across  a  room  to  be 
absorbed  by  the  frosty  window  panes.  Outdoors  it 
may  go  into  interminable  space.  The  heat  which  we 
radiate  is  often  compensated  by  the  heat  radiated  to  us. 
Two  rooms  may  have  the  same  temperature  as  deter- 
mined by  thermometers  hanging  in  central  locations,  but 
one  may  seem  much  cooler  than  the  other  if  it  has  cool 
walls  and  windows.  In  this  room  we  radiate  heat  which 

24 


370 


HUMAN    PHYSIOLOGY 


is  not  returned;  in  the  other  the  give  and  take  are  more 
nearly  equal. 

Radiation  is  hindered  by  humidity.  All  things  cool 
down  more  rapidly  at  nightfall  if  the  sky  is  cloudless 
and  the  air  dry  than  when  it  is  foggy.  It  is  in  regions 
of  low  humidity,  like  Arizona,  that  the  greatest  differ- 
ences between  night  and  day  occur.  But  the  humidity 
that  limits  radiation  favors  conduction  and  this  gives 
cold,  damp  air  an  added  chill.  Winter  on  our  Atlantic 


FIG.  69. — Cats  in  hot  and  cold  environments.      (See  text.) 

coast  is  declared  to  be  more  trying  than  the  same  season 
inland  although  the  thermometer  goes  much  lower  in  the 
interior.  For  the  same  reason  that  water  feels  colder 
than  air  of  the  same  temperature  moist  air  feels  colder 
than  dry.  High  humidity  makes  us  warmer  in  summer 
and  colder  in  winter. 

In  discussing  the  conditions  affecting  the  metabolism 
we  have  shown  how  the  body  meets  external  cold.  The 
skeletal  muscles  are  called  into  extra  activity.  Without 
special  coverings  to  keep  the  heat  pent  in  the  body  one 
cannot  be  still.  A  rise  in  the  metabolism  is  secured 
which  is,  as  we  have  insisted,  essentially  a  case  of  exer- 


MAINTENANCE    OF   THE   BODY    TEMPERATURE      371 

else.  We  have  saved  ourselves  to  a  great  extent  from 
the  need  of  such  an  increase  by  adapting  our  clothing  to 
the  weather.  Animals  do  the  same  thing  by  posture. 
On  a  hot  summer  day  a  cat  may  be  found  stretched  at 
full  length  with  the  tail  extended  and  the  paws  well 
apart.  In  this  position  the  fur  along  the  ventral  surface 
is  opened  to  the  air. 

On  a  cold  day  the  cat  squats  with  its  paws  pressed 
against  the  body,  the  tail  laid  alongside,  and  the  ventral 
aspect  covered  in  from  the  air.  The  surface  exposure 
cannot  be  one-half  of  what  it  is  in  the  extended  attitude. 
Rabbits  generally  keep  the  squatting  position  and  its 
value  is  shown  by  the  fact  that  they  readily  succumb  to 
cold  when  they  are  prevented  from  doing  so.  This  is 
the  more  striking  because  rabbits  endure  the  most  severe 
climates. 

Temperature  Regulation  during  Exercise. — We  have 
considered  thus  far  the  problem  of  temperature  main- 
tenance with  varying  outward  conditions.  We  have 
found  that  within  wide  limits  this  is  a  matter  of  restrict- 
ing or  facilitating  the  loss  of  heat  from  the  body  rather 
than  a  regulation  of  the  rate  of  heat  production.  When 
a  man  who  has  been  resting  rises  and  sets  out  on  a  brisk 
walk  we  have  a  distinct  case  to  analyze.  External 
factors  may  be  unchanged  but  the  metabolism  is  much 
increased.  If  the  body  is  to  be  successful  in  avoiding  a 
fever  it  must  make  adjustments  to  dissipate  the  extra 
Calories  which  it  is  generating. 

Two  factors  are  recognized  which  are  operative  both  in 
the  adaptation  to  warm  surroundings  and  to  muscular 
activity.  These  are  the  dilation  of  the  cutaneous  vessels 
and  the  secretion  of  sweat.  But  we  must  note  a  differ- 
ence between  the  skin  which  is  flushed  by  contact  with 
warm  air  and  that  which  glows  with  exercise.  In  the 
former  the  blood  is  not  moving  at  an  unusual  rate 
through  the  dilated  vessels;  in  the  latter  the  velocity  is 
presumably  increased  somewhat  with  the  general  quick- 
ening of  the  circulation. 


372  HUMAN    PHYSIOLOGY 

Two  other  factors  are  discovered  during  muscular 
activity  which  are  not  clearly  present  during  simple 
exposure  to  warm  air.  One  of  these  is  deep  breathing. 
It  is  only  certain  species  of  animals  which  pant  when  they 
are  warmed  from  without  but  all  animals  breathe  deeply 
when  they  are  producing  uncommon  quantities  of  heat 
internally.  The  primary  service  of  the  increased  ventila- 
tion of  the  lungs  is  to  provide  more  oxygen  and  remove 
more  carbon  dioxid  but  the  discharge  of  heat  is  promoted 
at  the  same  time. 

Another  means  of  shaking  off  heat  during  exercise  is 
found  in  the  constant  shifting  of  contact  between  the 
skin  and  the  air.  The  effect  is  that  of  a  breeze.  It  is 
plain  that  a  man  who  is  riding  a  bicycle,  running,  or 
walking  enjoys  this  favoring  condition.  The  same  is 
true  in  a  measure  of  the  man  who  is  standing  in  one  spot 
and  swinging  his  arms;  he  profits  by  the  fanning  of  his 
skin.  The  air  which  lies  against  it  at  one  moment  and 
has  become  warm  and  moist  is  replaced  a  moment  later 
by  cooler  and  drier  air.  An  important  detail  is  the 
pumping  of  air  from  within  the  clothing  and  the  sub- 
stitution of  fresh  portions.  When  one  is  resting,  the  air 
in  contact  with  the  skin  and  under  the  clothing  is  warm 
and  nearly  saturated.  Someone  has  said  that  "with  the 
exception  of  head  and  hands  we  live  in  a  tropical  climate." 

Fever. — The  body  temperature  will  rise  above  the 
standard  if  heat  production  is  increased  without  a  com- 
pensating increase  in  heat  loss.  It  will  also  rise  with  a 
uniform  heat  production  if  heat  loss  is  interfered  with. 
The  fever  of  the  Cornish  miners,  to  which  we  have  re- 
ferred, illustrates  more  particularly  the  second  difficulty. 
The  metabolism  was  not  too  high  to  be  offset  by  thermo- 
taxis  if  the  external  conditions  had  been  reasonable. 
The  combination  of  high  temperature  with  high  humidity 
led  to  an  accumulation  of  heat  in  the  tissues. 

It  may  be  said  of  fever  in  general  that  it  is  not  so  much 
the  result  of  high  metabolism  as  of  a  failure  of  the 
mechanisms  of  heat  dissipation.  It  is  true  that  the 


MAINTENANCE    OF   THE   BODY    TEMPERATURE      373 

heat  production  of  the  restless  patient  with  his  rapid 
heart  and  breathing  is  higher  than  it  would  be  if  he  were 
well  and  lying  comfortably  in  bed.  But  it  is  by  no  means 
so  high  as  it  might  be  if  he  were  well  and  taking  exercise. 
The  best  statement  that  we  can  make  regarding  fever  is 
that  the  trouble  is  with  the  nerve  centers  through  which 
the  balance  between  thermogenesis  and  thermotaxis  is 
normally  maintained.  When  a  fever  is  holding  a  steady 
course  the  balance  is,  in  fact,  preserved  as  delicately  as 
in  health.  It  is  the  false  standard  to  which  the  system  is 
held  which  is  the  real  characteristic. 

It  will  be  well  to  point  out  here  something  which  may 
have  been  gathered  from  statements  made  earlier  in  the 
book.  This  is  the  fact  that  our  sensations  are  altogether 
unreliable  as  indicators  of  the  body  temperature.  They 
are  entirely  dependent  upon  the  skin.  If  the  surface  of 
the  body  is  warm  we  say  that  we  are  warm.  The  glow 
produced  by  alcohol  furnishes  this  impression,  but  what 
is  actually  sensed  on  such  occasions  is  the  flux  of  heat 
from  within  the  body  to  the  exterior.  The  subject  feels 
warm  because  heat  is  escaping  through  his  skin.  Sub- 
normal temperatures  have  often  been  recorded  when  the 
influence  of  alcohol  has  been  coupled  with  exposure  to 
cold. 

Conversely,  when  the  circulation  in  the  skin  is  abnor- 
mally reduced  the  feeling  is  likely  to  be  one  of  chilliness,  but 
the  restriction  of  heat  loss  at  such  a  time  may  lead  to  a  rise 
of  the  internal  or  true  body  temperature.  In  the  chills 
of  malaria  this  is  just  what  happens.  The  patient  can 
scarcely  be  made  to  feel  comfortably  warm  because  of  the 
severe  constriction  of  the  vessels  of  the  skin.  The  sur- 
face cooling  leads  to  reflex  shivering  and  the  muscles 
produce  extra  heat  which  is  retained  in  the  internal  organs. 
The  temperature  mounts  high  above  the  normal  while 
the  sufferer  appeals  for  more  covering.  Later,  the  skin 
becomes  flushed  and  the  sensation  is  one  of  rising  tem- 
perature, though  the  fact  is  that  the  system  is  parting 
with  its  stored  heat  and  returning  to  the  normal  state. 


CHAPTER  XXVIII 
INTERNAL  SECRETIONS 

When  we  think  of  the  means  by  which  coordination  is 
secured  in  the  body  the  picture  of  the  nervous  system  at 
once  rises  before  us.  It  is  true  that  the  quick  adaptive 
reactions  by  which  emergencies  are  met  proceed  from  the 
flight  of  nerve-impulses.  But  it  is  a  fact  much  better 
appreciated  now  than  a  few  years  ago  that  chemical 
compounds  borne  from  place  to  place  by  the  blood  have  a 
great  deal  to  do  with  internal  adjustments.  The  subject 
may  be  called  that  of  the  internal  secretions  or,  using  a 
term  we  have  already  employed,  the  hormones. 

A  hormone  is  a  substance  produced  in  a  definite  locality 
but  having  its  effect  elsewhere,  perhaps  at  a  great  distance. 
At  least  two  examples  which  have  been  mentioned  should 
be  recalled.  One  is  the  secretin  formed  in  the  lining 
of  the  duodenum  and  taking  effect  upon  the  pancreas  and 
other  digestive  glands.  A  different  hormone,  gastric 
secretin,  is  believed  to  arise  in  the  lining  of  the  stomach 
under  the  influence  of  secretagogues  and  to  be  the  chief 
cause  of  the  late  formation  of  gastric  juice  in  the  normal 
period  of  digestion. 

Another  internal  secretion  of  which  it  was  necessary 
to  speak  in  an  earlier  chapter  is  that  contributed  by  the 
pancreas  to  the  circulation  and  essential  to  the  oxidation 
of  sugar.  The  want  of  this  hormone  is  responsible  for 
diabetes.  We  must  now  add  to  these  examples  several 
others.  It  may  be  said  at  the  outset  that  a  greater 
degree  of  confusion  exists  in  this  department  of  physiology 
than  in  any  other.  This  is  partly  owing  to  the  newness 
of  the  recognition  of  hormones  but  is  partly  inherent  in  the 
subject  itself.  Fresh  discoveries  are  being  reported  and 

374 


INTERNAL    SECRETIONS  375 

still  more  are  to  be  anticipated.  But  down  to  the  present 
time  the  observations  have  constantly  increased  the 
difficulty  of  the  whole  matter. 

It  may  fairly  be  claimed  that  every  kind  of  tissue  may 
give  rise  to  hormones.  That  is,  it  may  be  argued  that 
the  tissues  differ  in  their  chemical  nature  and  metabolism 
so  distinctly  that  each  one  must  have  products  which 
no  other  can  evolve.  The  normal  composition  of  the 
blood  must  depend  on  the  blending  of  all  these  innumer- 
able contributions.  It  is  curious  to  reflect  that  the  Greek 
masters  in  medical  science  taught  that  health  was  founded 
on  the  right  combination  of  chemical  principles,  the 
''four  humors,"  in  the  body.  We  are  coming  to  believe 
in  our  own  age  that  many  disturbances  of  health  are 
actually  due  to  excesses  or  deficiencies  of  internal  secre- 
tions, a  view  that  obviously  recalls  the  ancient  one. 
While  the  formation  of  hormones  may  be  theoretically 
universal,  there  are  some  organs  which  exhibit  it  most 
conspicuously  and  of  these  we  have  now  to  give  brief 
accounts. 

The  Thyroid. — The  thyroid  gland,  or  thyroid  body,  is 
in  the  neck  between  the  larynx  above  and  the  breast 
bone  below.  There  are  two  lobes  united  by  an  isthmus 
which  crosses  in  front  of  the  trachea.  The  normal  thy- 
roid is  small  and  not  easily  noticeable  from  the  surface, 
but  there  are  many  thin  subjects  in  whom  its  form  is 
quite  apparent.  It  may  be  greatly  enlarged  and  when  this 
is  so  it  forms  the  disfiguring  swelling  known  as  a  goiter. 
The  enlargement  may  not  be  attended  by  other  than  local 
symptoms,  but  in  many  cases  it  is  the  central  element  in 
a  serious  disease. 

We  have  said  that  extracts  of  the  thyroid  produce 
nervousness,  emaciation,  and  palpitation  of  the  heart 
when  given  to  human  subjects.  The  enlarged  gland  may 
deal  out  to  the  circulation  abnormal  quantities  of  an 
internal  secretion  capable  of  just  these  effects.  The  result 
is  much  as  though  the  extracts  were  given  by  the  mouth. 
In  bad  cases  an  odd  symptom  is  a  bulging  of  the  eyes. 


376 


HUMAN    PHYSIOLOGY 


As  there  may  be  too  much  of  the  thyroid  product,  so 
there  may  in  other  individuals  be  too  little.  The  effects 
are  consistent  with  those  that  have  been  described. 
Instead  of  nervousness  there  is  apathy  and  dullness; 
instead  of  emaciation  there  is  corpulence.  A  peculiar 
characteristic  of  the  person  with  insufficient  thyroid  is 
the  overgrowth  of  the  connective  tissue  beneath  the 
skin.  This  destroys  the  grace  of  all  the  contours  and 
particularly  affects  the  features,  making  them  heavy 
and  uncouth.  The  disease  is  called  myxedema. 

The  symptoms  we  have  just  re- 
ferred to  are  those  shown  by  a  person 
who  has  formerly  had  a  normal  thy- 
roid but  has  lost  its  support  more  or 
less  completely.  Another  possibility 
is  that  the  thyroid  may  be  inactive 
from  birth.  This  has  a  shocking  con- 
sequence: the  child  remains  in  a  state 
of  arrested  development  both  phys- 
ically and  mentally.  It  is  said  to 
be  a  cretin.  It  is  not  only  dwarfed 
but  ill-proportioned,  having  a  heavy 
head  and  abdomen  and  weak  muscles, 
thy-  The  development  of  a  cretin  can  be 
greatly  assisted  by  adding  to  its  food 
at  an  early  period  some  of  the  dried 
substance  of  the  thyroids  of  animals.  The  impulse  given 
to  growth  and  the  approximation  to  a  normal  type  under 
the  influence  of  thyroid  feeding  are  among  the  most 
wonderful  demonstrations  of  modern  medicine. 

The  main  mass  of  the  thyroid  is  composed  of  a  tissue 
which  suggests  that  of  a  gland,  such  as  the  pancreas, 
but  with  an  important  difference.  The  cells  are  ob- 
served to  surround  recesses,  as  in  the  true  gland,  and  the 
recesses  seem  to  be  distended  with  a  secretion,  but  there 
are  no  ducts.  Any  active  product  must  leave  the  seat 
of  its  formation  by  the  lymph  or  the  blood.  The  force 
of  the  term  internal  secretion  comes  home  to  one  while 


FIG.   70.— The 
roid   in  its  relation 
the  trachea. 


to 


INTERNAL    SECRETIONS 


377 


looking  at  such  an  arrangement.  Four  small  nodules 
of  a  distinctive  tissue,  unlike  that  of  the  main  thyroid, 
are  found  imbedded  in  it  or  close  by.  These  kernels  are 
the  parathyroids,  and  there  is  no  doubt  that  they  send 
out  a  valuable  hormone.  In  their  absence  convulsive 
disturbances  of  the  skeletal  muscles  occur. 

The  Adrenals. — These  are  two  small  bodies  which  are 
placed  above  the  kidneys.  The  fact  that  they  are  neces- 
sary to  life  has  long  been  known.  If  they  are  wasted  by 
disease,  as  by  a  localized  tuberculosis,  one  or  more  in- 
dispensable hormones  seem  to  be  lost.  The  victim  grows 


FIG.  71. — The  adrenals  (n,ri),  surmounting  the  kidneys  and  close 
under  the  diaphragm. 

weak,  suffers  from  incessant  nausea,  and  dies  as  though 
from  exhaustion.  An  incident  of  the  decline  is  a  dark 
pigmentation  of  the  skin  which  has  given  the  derange- 
ment the  name  of  Addison's  bronze  disease.  Dogs  de- 
prived of  the  adrenals  soon  die. 

An  extract  of  the  adrenal  body  has  extremely  active 
properties,  but  it  has  not  found  its  chief  use  in  the  cor- 
rection of  Addison's  disease,  as  might  be  inferred  from 
the  account  of  the  thyroid.  The  adrenal  extract  is  sold 
under  the  name  of  adrenalin.  It  contains  a  substance 
which  can  be  isolated  and  which  is  responsible  for  most 
of  the  powers  of  the  extract;  it  is  called  adrenin  or  epi- 


378  HUMAN    PHYSTOLOGY 

nephrin.  It  is  applied  to  wounds  to  check  bleeding,  for 
one  of  its  effects  is  to  cause  an  intense  contraction  of 
small  blood-vessels.  This  has  made  adrenalin  valuable 
to  surgeons  for  work  on  the  eye,  nose,  and  throat. 

It  has  lately  become  probable  that  we  must  distinguish 
between  two  functions  exercised  by  the  adrenal  bodies: 
they  have  an  obscure  relation  to  the  continued  welfare 
of  the  system  as  a  whole  and,  in  addition,  an  occasional 
or  emergency  function.  This  is  performed  under  excite- 
ment. In  Chapter  XII  we  made  the  point  that  emotion 
is  a  form  of  exercise  and  accompanied  by  an  extensive 
discharge  of  impulses  along  various  nerves.  It  has  been 
shown  that  among  these  streams  of  impulses  there  are 
some  which  reach  and  arouse  to  unusual  activity  the 
adrenal  bodies.  It  will  be  well  to  describe  an  experi- 
ment which  has  given  proof  of  this  effect. 

Very  delicate  tests  for  the  presence  of  adrenin  in 
fluids  have  been  made  available.  One  of  these  tests 
may  be  applied  to  a  sample  of  blood  taken  painlessly 
from  the  veins  of  a  cat,  and  there  will  usually  be  no  sign 
that  adrenin  is  present.  If  the  test  is  repeated  after  the 
cat  has  been  excited  by  seeing  a  dog  the  secretion  will 
have  made  its  appearance.  We  conclude  that  one  feature 
of  the  emotional  crisis  was  a  rapid  discharge  from  the 
adrenal  bodies  of  a  hormone  which  they  ordinarily  supply 
only  in  minute  quantities.  When  this  discovery  was 
made  it  was  natural  to  inquire  whether  the  adrenin  could 
serve  any  useful  purpose.  The  question  has  been  an- 
swered in  the  affirmative. 

It  has  been  found  that  the  contraction  of  the  blood- 
vessels which  occurs  under  the  influence  of  adrenin  is  not 
uniformly  distributed.  Some  areas  are  affected  more  than 
others.  The  most  marked  constriction  takes  place  in  the 
abdominal  viscera.  Meanwhile  the  vessels  of  the  lungs, 
the  heart's  walls,  and  the  skeletal  muscles  are  not  con- 
tracted at  all.  There  will  be  no  interference  with  the 
pulmonary  circulation  and  a  positive  promotion  of 
blood-flow  through  the  muscles.  A  great  master  of 


INTERNAL    SECRETIONS  379 

physiology  has  interpreted  the  reaction  for  us  from  the 
point  of  view  of  its  value  to  the  individual. 

An  emotional  occasion  is  an  occasion  for  action.  This 
was  universally  true  under  primitive  conditions  of  life 
among  men  and  it  is  true  in  the  lives  of  the  lower  animals. 
Fear  is  the  prelude  to  flight.  Anger  is  the  impulse  to 
attack.  Under  the  experience  of  pain  there  are  efforts  to 
escape  from  the  cause  of  suffering  where  this  is  possible. 
There  is  need  for  the  fullest  command  of  all  bodily 
resources  in  these  crucial  exigencies.  It  can  be  demon- 
strated that  adrenin  helps  to  realize  such  a  command. 

Take  first  the  effect  upon  the  distribution  of  the  blood 
noted  above.  It  is  clearly  favorable  to  the  maximum 
activity  of  the  muscles.  The  progress  of  digestion  and 
the  other  processes  which  may  be  going  on  in  the  ab- 
dominal organs  can  be  held  in  abeyance  for  the  time 
being.  The  most  vital  needs  are  given  the  precedence. 
At  the  same  time  there  is  likely  to  be  some  rise  of  arterial 
pressure  and  this  accelerates  the  circulation  through  the 
muscles.  Until  the  actual  struggle  is  under  way  the 
skin  may  be  pale  rather  than  flushed;  this  indicates  the 
greatest  possible  concentration  of  the  blood-flow  in  the 
vessels  of  the  motor  organs. 

Another  service  of  adrenin  is  known  to  be  a  postpone- 
ment of  fatigue.  A  very  small  addition  of  the  adrenal 
principle  to  the  blood  of  an  animal  whose  strength  is 
flagging  may  give  a  renewed  command  of  the  muscles. 
The  gain  is  partly  in  the  way  of  better  end-plate  trans- 
mission, but  probably  there  are  other  points  of  applica- 
tion. It  is  now  believed  that  the  secret  of  "the  strength 
of  desperation"  is  largely  in  the  timely  discharge  of 
adrenin  into  the  blood-stream. 

Still  another  result  of  the  emotional  disturbance  is  a 
rapid  transformation  of  the  liver  glycogen  into  sugar 
which  at  once  enters  the  blood.  In  Chapter  XXIII  it 
has  been  stated  that  the  resulting  hyperglycemia  may  be 
so  marked  as  to  cause  the  appearance  of  sugar  in  the 
urine.  The  loss  of  the  carbohydrate  cannot  be  of  any 


380  HUMAN    PHYSIOLOGY 

use,  but  the  temporary  concentration  of  the  sugar  in 
the  blood  may  be  purposeful.  The  wise  interpreter 
whose  exposition  we  are  following  points  out  that  extra 
fuel  of  the  preferred  kind  is  thus  offered  to  the  muscles. 
So  they  are  made  unusually  responsive  to  stimulation 
through  their  nerves  at  the  same  time  that  they  are 
provided  with  a  liberal  allowance  of  sugar  for  oxidation. 

An  odd  alteration  in  the  blood  which  can  be  noted 
directly  after  excitement  is  a  shortening  of  the  time  re- 
quired for  coagulation.  The  reduction  is  very  marked. 
The  suggestion  is  that  an  exciting  situation  is  one 
which  may  be  followed  by  conflict  and  loss  of  blood. 
The  chances  of  victory  and  survival  will  be  better  for  the 
combatant  whose  blood  most  promptly  staunches  its 
own  flow. 

Adrenin  plays  a  part  in  securing  all  the  valuable  adapt- 
ive changes  that  have  been  enumerated.  It  would  not 
be  wholly  correct  to  say  that  it  can,  without  assistance, 
cause  them  all,  for  its  influence  is  intimately  combined 
with  others  of  a  nervous  order.  There  are  certain  nerve 
paths  in  the  body  which  can  .be  stimulated  with  the 
result  that  the  following  effects  are  produced:  quickening 
of  the  heart,  dilation  of  the  pupils,  contraction  of  the 
abdominal  blood-vessels,  erection  of  the  hairs,  sweating, 
and  the  discharge  of  adrenin.  Adrenin  itself  can  produce 
most  of  the  associated  reactions.  It  may  be  conceived 
that  during  the  experience  of  emotion  impulses  from  the 
brain  traverse  these  paths,  adrenin  is  added  to  the  blood, 
and  the  hormone  perpetuates  and  extends  effects  which 
were  at  first  nervous  in  origin. 

It  may  be  remarked  that  although  these  bodily 
changes .  are  admirably  suited  to  the  needs  of  animals 
and  cave  men,  they  are  not  so  well  suited  to  the  restrained 
life  of  civilization.  When  we  experience  emotion  we  try 
to  refrain  from  manifesting  it  violently.  It  is  not  un- 
likely that  the  physical  accompaniments  are  harmful 
when  no  application  of  them  is  made.  Yet,  as  was  said 
in  Chapter  XII,  a  life  deficient  in  emotion  is  a  life  lacking 


INTERNAL    SECRETIONS  381 

important  elements  of  training  as  well  as  of  interest. 
The  conclusion  is  that  emotion  should  not  be  excluded, 
even  though  that  were  possible,  but  that  it  should  be 
given  reasonable  expression — made  a  motive  for  action. 
When  we  work  off  our  anger  or  express  our  happiness  in 
deeds  we  are  true  to  our  remote  biologic  inheritance. 

Other  Organs  of  Internal  Secretion. — One  of  these 
which  has  claimed  a  good  deal  of  attention  recently  is 
the  hypophysis.  This  is  a  small  but  compound  structure 
united  by  a  stalk  to  the  under  surface  of  the  brain.  It  is 
lodged  in  a  hollow  of  the  sphenoid  bone.  The  removal  of 
an  organ  so  situated  requires  a  severe  operation,  but  it 
has  been  accomplished  many  times.  The  loss  of  the 
entire  hypophysis  is  fatal  after  a  short  interval.  There  is 
reason  to  think  that  it  is  a  producer  of  hormones.  When 
it  is  diseased  development  is  perverted,  the  resulting 
abnormalities  being  mainly  in  the  shape  of  the  bones. 

When  one  looks  at  the  pictures  which  ha've  been  made 
of  persons  with  disease  of  the  hypophysis  or  the  thyroid 
one  is  inclined  to  think  that  a  great  many  individuals 
show  in  slight  degree  the  departures  from  the  normal 
which  are  carried  to  an  extreme  in  these  selected  cases. 
We  constantly  see  faces  which  are  strangely  moulded 
and  which  do  not  seem  to  register  the  true  character  and 
intelligence  of  the  man  or  woman.  The  underlying 
condition  may  well  be  an  excess  or  a  lack  of  some  inter- 
nal secretion. 

The  Reproductive  Glands. — We  have  seen  'that  the 
same  organ  may  send  products  to  the  exterior  and  to  the 
.circulation.  This  is  true  of  the  pancreas.  While  it 
prepares  a  valuable  digestive  juice  it  is  also  making  a 
contribution  to  the  blood.  We  find  a  corresponding  state 
of  things  to  hold  for  the  testes  and  the  ovaries.  The 
unique  function  of  these  organs  is  to  detach  the  germ-cells 
which  shall  originate  a  new  generation,  but  they  are  not 
without  influence  upon  the  organisms  which  bear  them. 

Removal  of  the  reproductive  glands  from  young 
animals  profoundly  modifies  the  course  of  their  de- 


382  HUMAN    PHYSIOLOGY 

velopment.  Sterility  is  only  one  result  among  many. 
The  contrast  between  the  ox  and  the  bull,  the  stallion 
and  the  gelding  is  a  familiar  one.  It  is  as  much  a  con- 
trast in  temperament  as  in  build.  Animals  without  the 
generative  organs  are  said  to  lose,  or  rather  never  to 
acquire,  the  secondary  sexual  characters.  In  man  these 
include  the  beard  and  the  large  larynx  which  accounts  for 
the  average  difference  of  an  octave  in  the  pitch  of  male 
and  female  voices.  After  maturity  has  been  attained 
the  changes  that  follow  the  operation  are  not  striking. 

One  of  the  marks  of  a  male  frog  is  a  bulbous  thumb. 
It  has  been  found  that  the  early  removal  of  the  testes 
prevents  the  development  of  this  character  and  that  it  is 
formed  within  a  short  time  after  the  grafting  into  the 
body  of  a  testis  from  another  frog.  There  could  hardly 
be  a  clearer  demonstration  of  the  power  of  hormones 
liberated  by  one  tissue  to  influence  the  metabolism  of 
another.  If  the  transmitted  effect  were  a  nervous  one 
it  would  make  a  difference  where  the  grafting  had  been 
done ;  in  fact,  it  makes  no  difference  at  all. 

Some  years  ago  it  was  argued  that  the  decline  of  the 
reproductive  system  in  advanced  life  might  have  much  to 
do  with  the  simultaneous  deterioration  of  other  organs, 
especially  the  brain  and  cord.  Vigor  and  efficiency  might 
be  prolonged,  it  was  thought,  by  introducing  into  the 
body  extracts  from  the  reproductive  glands  of  animals. 
Trials  were  made  upon  senile  subjects,  who  reported  some 
stimulation.  But  the  results  fell  far  short  of  the  reju- 
venation that  had  been  hoped  for  and  such  as  were 
described  have  been  credited  for  the  most  part  to  sug- 
gestion. There  has  been  no  widely  approved  use  of  the 
testicular  extracts  since  the  failure  of  the  so-called  Elixir 
of  Life.  Ovarian  extracts  have  been  employed  with  ad- 
vantage to  abate  distressing  symptoms  which  follow  the 
removal  of  the  female  organs. 

The  Spleen. — This  large  organ  is  of  a  type  which  might 
lead  to  the  expectation  that  it  could  be  shown  to  have 
an  internal  secretion.  The  evidence,  however,  has  not 


INTERNAL    SECRETIONS  383 

definitely  supported  the  natural  assumption.  The  spleen 
has  a  large  blood-supply  and  is  periodically  contracted 
and  enlarged  as  though  it  were  actively  engaged  in  some 
way.  But  it  has  been  found  that  animals  and  men  sur- 
vive its  removal,  provided  they  rally  from  the  immedi- 
ate effects  of  the  operation.  Obscure  differences  in  the 
composition  of  the  blood  have  been  noted  in  such  surviv- 
ing animals,  and  we  have  previously  said  that  there  is 
sometimes  a  lessened  destruction  of  the  red  corpuscles. 

The  Thymus. — This  is  an  organ  below  the  thyroid  and 
behind  the  upper  part  of  the  breast  bone.  It  is  very  large 
in  embryonic  life  and  through  infancy,  gradually  dimin- 
ishing later  until  only  scattered  remnants  of  its  tissue 
are  left.  It  is  the  "  neck  sweetbread  "  of  the  market.  It 
is  probable  that  the  thymus  has  some  regulating  effect 
in  the  processes  of  growth  and  development.  The  same 
has  been  claimed  for  the  pineal  body,  an  outgrowth  from 
the  dorsal  surface  of  the  brain-stem. 

Attention  has  been  called  recently  to  the  similarity 
between  the  effects  produced  by  feeding  the  substance  of 
several  organs  of  internal  secretion  and  the  influence  of 
the  bodies  we  have  called  vitamins.  The  hormones  from 
the  thyroid,  the  thymus,  the  hypophysis,  and  the  pineal 
body  may  enter  into  the  nutrition  of  various  tissues  in  much 
the  same  helpful  way  as  these  accessory  compounds  in 
the  diet.  The  suggestion  has  also  been  made  that  the 
vitamins  are  particularly  useful  to  the  glands  of  internal 
secretion.  These  organs  may  transform  the  vitamins  of 
the  food  into  hormones  needed  by  the  tissues. 

There  is  an  aspect  of  this  subject  which,  in  the  present 
state  of  our  knowledge,  adds  greatly  to  its  difficulty  and 
obscurity.  This  is  the  circumstance  that  the  organs  of 
internal  secretion  have  reciprocal  relations  of  extreme 
complexity.  One  hormone  may  be  auxiliary  to  another 
or  it  may  be  antagonistic.  The  future  treatment  of  the 
matter  will  be  shaped  in  conformity  with  a  vast  number  of 
facts  of  this  kind  and  its  trend  can  hardly  be  foreseen. 


CHAPTER  XXIX 
SOME  MATTERS  OF  HYGIENE 

A  chapter  on  the  Hygiene  of  the  Nervous  System  has 
found  a  place  in  this  book  and  also  one  on  the  Hygiene  of 
Nutrition.  Other  suggestions  regarding  the  right  use 
of  the  body  have  been  dropped  from  time  to  time.  It 
may  be  well  in  conclusion  to  assemble  some  of  the  funda- 
mental principles  in  the  form  of  a  summary.  When  this 
is  undertaken  it  is  a  distinct  advantage  to  have  all  the 
topics  which  we  have  treated  as  a  background — to  cor- 
relate our  hygiene  with  our  physiology. 

Vitalism  and  Mechanism. — Living  matter  is  sharply 
distinguished  in  many  ways  from  lifeless.  The  con- 
trasts are  so  evident  that  men  of  science  formerly  be- 
lieved that  very  different  principles  must  be  effective 
in  the  two  states.  Organisms  were  supposed  to  tran- 
scend some  of  the  limitations  of  inorganic  bodies.  The 
teaching  that  they  are  thus  superior  to  what  are  called 
the  laws  of  nature  is  known  as  Vitalism.  The  contrary 
doctrine,  that  they  are  strictly  limited  by  these  laws, 
that  they  are  machines  transforming  energy  which  they 
neither  create  nor  destroy,  is  called  Mechanism. 

We  have  seen  that  the  experiments  made  in  labora- 
tories for  the  study  of  metabolism  show  that  animals  and 
men  are  rigidly  subject  to  the  principle  of  the  conserva- 
tion of  energy.  The  modern  tendency  has  been  to  em- 
phasize this  subjection  to  fixed  limitations,  and  it  is  certain 
that  experimental  progress  has  been  based  almost  wholly 
upon  faith  in  its  validity.  Our  objective  studies  must  be 
conducted  upon  organisms  or  parts  of  organisms  in  the 
hope  that  we  may  find  uniform  responses  to  the  condi- 
tions we  establish.  If  we  cannot  find  the  regularity  of 

384 


SOME    MATTERS    OF    HYGIENE  385 

behavior  which  is  the  characteristic  of  a  mechanism  we 
can  learn  nothing  that  is  significant. 

At  the  same  time,  it  may  be  acknowledged  that  the 
early  mechanists  were  cocksure  and  oversanguine  in  the 
expectation  that  they  could  analyze  all  the  reactions  of 
animals  with  ease.  Animals  may  be  machines,  but  they 
are  inconceivably  complex  and  correspondingly  removed 
from  ready  comparison  with  machines  of  human  con- 
struction. The  most  baffling  complexity  is  evident  in 
the  constitution  of  every  cell,  and  when  cells  are  associated 
in  enormous  numbers  the  difficulty  of  making  predictions 
in  regard  to  the  capacities  of  the  organism  is  increased 
according  to  a  mathematical  formula. 

In  the  light  of  all  that  is  known  we  may  choose  to 
emphasize  either  the  resemblance,  of  the  organism  to  a 
machine  or  its  dissimilarity.  One  may  be  a  strict 
mechanist  or  a  "  Neo-vitalist "  as  one  assumes  the 
former  attitude  or  the  latter.  The  neo-vitalist  is  im- 
pressed with  the  wonder  and  mystery  of  life,  but  so  far 
as  he  looks  for  additions  to  our  knowledge  he  approves 
the  methods  and  deductions  of  the  mechanist.  The 
mechanist  must  also  have  his  moods  of  marvelling  and  so 
the  two  are  not  so  far  apart  as  is  sometimes  assumed. 
The  ultimate  question  of  the  relationship  of  consciousness 
to  organic  matter  seems  unanswerable. 

This  discussion  of  the  scientific  point  of  view  has  been 
introduced  because  it  bears  directly  upon  one's  estimate 
of  hygiene.  If  the  body  is  in  any  real  sense  a  mechanism 
it  is  fair  to  insist  that  it  be  cared  for  systematically. 
If  it  is  superior  to  all  the  limitations  of  a  mechanism  it 
may  be  superfluous  to  give  time  and  thought  to  its  care. 
" Living  on  one's  nerve"  may  be  noble  in  this  case  but 
it  is  clearly  reprehensible  according  to  the  mechanistic 
conception.  We  shall  adhere  provisionally  to  the  idea 
that  the  body  is  a  machine,  unique  in  its  power  of  self- 
repair,  but  so  limited  in  this  and  other  respects  as  to 
impose  the  obligation  of  careful  conduct  upon  the 
individual. 

25 


386  HUMAN    PHYSIOLOGY 

Mental  States. — At  the  present  time  much  is  said  of 
the  importance  of  mental  states  for  the  maintenance 
of  health  and  for  its  restoration  when  it  has  been  lost. 
Do  we  deny  the  force  of  this  claim  when  we  take  a 
mechanistic  position?  We  deny  certain  sweeping  as- 
sertions but  we  continue  to  assent  to  a  moderate  applica- 
tion of  the  teaching.  There  is  a  clear  correlation  between 
mental  serenity  and  the  harmony  of  physiologic  activities. 
There  is  room  for  argument  as  to  which  is  primary  and 
which  secondary  in  a  given  case.  Most  men  take  it  for 
granted  that  the  mind  reflects  the  condition  of  the 
body,  and  the  body  the  mental  content.  It  is  prudent  to 
avoid  the  metaphysical  discussion  and  lay  stress  only 
upon  the  parallel  between  the  two. 

If  we  can  lead  a  man  who  is  obviously  ill  and  depressed 
into  a  confident  and  benevolent  frame  of  mind  it  is  likely 
enough  that  many  of  his  symptoms  may  be  relieved. 
The  turning  point  may  be  passed  and  his  recovery 
go  forward  from  that  hour.  Since  this  practical  possi- 
bility exists  it  is  not  important  to  decide  whether  the 
mental  state  is  causative  or  whether  it  is  symptomatic. 
It  is  held  by  some  to  occasion  the  desired  adjustment  of 
the  nervous  system  and  by  others  merely  to  attend  it. 
Whichever  it  is  it  gives  us  something  to  work  for  in  con- 
tending against  illness  in  ourselves  or  others.  It  need 
not  lessen  our  respect  for  material  measures  of  treatment. 

A  word  should  be  said  here  about  the  various  schools  of 
therapeutics  which  exist  side  by  side  and  seem  to  minister 
with  such  success  to  human  infirmity.  It  may  be 
affirmed  that  each  system  succeeds,  so  far  as  it  is  found 
to  do  so,  by  reason  of  the  merit  that  is  in  it.  This  is 
equally  true  whether  the  practice  is  mental,  mechanical, 
or  pharmacological.  Each  is  wrong  in  denying  virtue 
to  its  competitors.  The  regular  practitioner  stands 
superior  to  all  who  have  allied  themselves  to  peculiar 
and  exclusive  systems,  recognizing  well  the  elements  of 
good  in  each  though  regretting  the  bigotry  which  their 
exponents  display.  He  is  constantly  blamed  for  with- 


SOME    MATTERS    OF    HYGIENE  387 

holding  his  endorsement  of  measures  which  have  proved 
valuable.  Such  measures  he  might  be  ready  to  com- 
mend if  he  were  not  thereby  committed  to  the  denials 
as  well  as  the  affirmations  of  his  rival. 

The  old  trust  in  drugs  is  not  widely  prevalent  to-day 
unless  among  rather  ignorant  people  who  form  the  clien- 
tele of  the  makers  of  patent  medicines.  A  good  many 
people  underestimate  the  occasional  utility  of  drugs. 
But  it  is  a  sound  principle  that  a  drug  is  for  a  definite 
emergency,  or  for  an  incurable  condition,  and  that  it  is 
not  to  be  substituted  for  hygienic  living.  As  was  said 
of  cathartics,  so  it  may  be  said  of  headache  cures,  cough 
syrups,  "  tonics,"  and  other  preparations  that  the  oc- 
casion for  their  use  should  likewise  be  an  occasion  for 
reflecting  how  a  recurrence  can  be  avoided. 

If  we  attempt  to  name  the  great  requisites  of  living  in 
health  and  efficiency  we  may  make  a  list  somewhat  as 
follows.  First  of  -all,  the  inheritance  must  be  sound. 
This,  unhappily,  lies  outside  the  choice  of  the  individual. 
He  has  it  to  consider  as  he  in  his  turn  contemplates 
becoming  a  father.  Second,  we  must  have  successful 
nutrition;  we  need  not  renew  our  discussion  of  this  sub- 
ject. Third,  the  activities  must  be  balanced  and  rest 
must  be  adequate.  Fourth,  the  environment  must  be 
wholesome  and  intellectually  stimulating. 

Exercise. — We  must  enlarge  upon  the  third  require- 
ment of  the  series.  It  introduces  at  once  the  topic  of 
muscular  exercise  which  could  not  be  properly  handled 
until  we  had  outlined  all  the  physiologic  functions.  It 
is  related  to  all  of  them.  The  skeletal  muscles  are  given 
peculiar  distinction  by  several  facts.  They  form  about 
half  the  entire  body  and  they  are  the  seat  of  a  very  large 
share  of  the  metabolism.  In  the  second  place,  they  are 
under  voluntary  control;  it  remains  for  us  to  say  how 
they  shall  be  employed.  We  can  drive  or  spare  other 
organs  also — for  example,  the  digestive  and  the  sweat 
glands — but  there  are  none  which  we  can  so  slight  and 
neglect  to  exercise  as  these. 


388  HUMAN    PHYSIOLOGY 

In  a  book  like  this  we  cannot  enter  into  questions  of  the 
particular  kinds  of  muscular  activity  appropriate  to 
people  of  various  ages  and  habits.  We  must  limit 
ourselves  to  broad  statements.  It  will  be  convenient  to 
speak  first  of  the  effects  of  exercise  upon  the  neuro- 
muscular  mechanism  itself  and  then  to  show  in  how  many 
ways  its  influence  is  extended  to  other  systems.  The 
object  of  exercise  is  sometimes  training  for  special  accom- 
plishments and  sometimes  simply  the  preservation  of  the 
general  health. 

The  most  familiar  fact  in  the  mind  of  the  schoolboy 
is  that  muscles  grow  with  use.  The  increase  is  said  not 
to  be  in  the  number  but  in  the  size  of  the  fibers.  The 
changes  which  accompany  contraction  are  of  a  destruc- 
tive kind,  but  it  seems  to  be  commonly  true  in  biology 
that  the  compensation  for  a  wasting  process  is,  in  a  vigor- 
ous tissue,  more  than  equal  to  the  original  loss  of  sub- 
stance. It  does  not  merely  recover  but  it  becomes 
larger  than  it  was  before.  Of  course  an  increase  in  the 
size  of  muscles  means  a  gain  in  strength,  but  we  should  be 
very  much  in  error  if  we  were  to  overlook  certain  other 
factors. 

It  is  altogether  probable  that  betterment  of  quality 
is  a  more  important  result  of  training  than  sheer  gain  in 
mass.  We  can  easily  think  of  persons  whose  muscles 
are  insignificant  in  appearance  but  whose  endurance  is 
remarkable.  Several  conditions  can  be  suggested  which 
they  probably  exemplify.  First,  their  muscles  are  supe- 
rior in  a  chemical  sense.  Second,  the  circulation  is 
advantageously  directed  to  give  them  support.  Third, 
the  blood  itself  comes  to  them  with  a  favorable  com- 
position— not  impaired  for  service  by  the  presence  of 
poisons  either  derived  from  the  intestine  or  from  the 
metabolism. 

Efficient  muscles  must  also  be  distinguished  by  efficient 
innervation.  The  best  possible  end-plate  transmission 
may  be  assumed.  Another  feature  may  be  a  more  general 
employment  of  the  units  than  is  secured  by  the  untrained 


SOME.  MATTERS    OF    HYGIENE  389 

subject.  It  may  be  that  a  muscle  which  seems  less 
strong  than  we  should  anticipate  in  view  of  its  size  is 
one  in  which  there  are  many  idle  fibers.  Finally,  the 
working  capacity  of  any  set  of  muscles  must  depend 
upon  the  organization  of  the  central  nervous  system  and 
the  manner  of  using  it. 

Let  us  emphasize  the  idea  just  advanced.  It  means 
that  efficiency  depends  upon  coordination,  for  coordina- 
tion is  secured  through  the  interrelations  of  the  neurons 
in  the  cord  and  the  brain.  The  attainment  of  skill  and 
ease  comes  with  the  establishment  of  these  associations. 
The  cerebellum  as  well  as  the  cerebrum  must  be  in- 
volved. Only  very  lately  has  it  been  appreciated  that 
the  afferent  as  well  as  the  efferent  mechanism  must  be 
credited  with  a  share  in  pushing  motor  resources  to  their 
limit.  The  matter  is  too  involved  to  be  presented  in  full 
but  a  paragraph  may  be  given  to  it. 

We  have  only  to  add  one  more  step  to  a  familiar  series. 
If  we  try  to  make  a  list  of  the  factors  on  which  the 
effective  power  of  a  muscle  depends  we  shall  parallel  the 
enumeration  above.  We  shall  think  first  of  the  muscle's 
own  size  and  nature.  Then  we  shall  recognize  the  limita- 
tion imposed  by  the  end-plates  and  then  the  dependence 
of  these  for  stimulation  upon  the  motor  centers  of  the 
cord.  These  are  played  upon  by  impulses  from  at 
least  two  sources:  those  that  come  from  the  receptors,  as 
in  the  production  of  simple  reflexes,  and  those  which 
come  from  the  cerebral  motor  cortex.  We  might  well 
mention  the  impulses  from  the  cerebellum  also. 

Now  a  spinal  center  may  be  supposed  to  transmit  im- 
pulses with  a  maximum  effect  when  it  is  beset  from  as 
many  angles  as  possible,  when  all  the  available  means  of 
excitation  are  combined  to  bear  upon  it.  The  same  will 
apply  to  the  motor  centers  of  the  cerebrum;  these  are  not 
self-stimulated,  but  depend  on  other  elements  to  induce 
reactions  through  them.  It  follows  that  muscles  must 
give  their  best  performance  when  the  governing  centers 
are  subjected  to  the  most  multiplied  stimulation  and  this 


390  HUMAN    PHYSIOLOGY 

includes  potentially  the  whole  receptor  system.  In 
other  words,  one  reason  for  the  power  of  the  athlete  is 
his  responsiveness  to  what  he  sees,  hears,  and  feels — the 
excellence  of  his  afferent  equipment. 

We  find  that  fatigue  is  greatly  delayed  when  we  are 
doing  something  that  we  enjoy.  Our  enjoyment  is 
probably  a  measure  of  the  richness  of  the  afferent  tides 
in  the  nervous  system  and  if  we  suppose  that  these  cur- 
rents are  applied  in  reflex  fashion  to  secure  innervation  of 
the  muscles  we  have  a  simple  explanation  of  our  own 
endurance.  Dancing  would  be  harder  work  than  sweep- 
ing a  room  if  the  comparison  were  not  entirely  de- 
stroyed by  the  superior  means  of  stimulation  which 
accompany  it. 

Other  Effects  of  Exercise. — We  may  now  turn  from  the 
value  of  exercise  as  a  way  to  improve  the  command  of  the 
motor  apparatus  and  mention  some  of  its  influences  upon 
other  systems.  We  may  pass  over  the  respiratory  fea- 
tures which  have  been  given  a  place  in  Chapter  XXI. 
Something  must  be  said  of  the  effects  upon  the  circula- 
tion. We  may  distinguish  conveniently  between  those 
that  relate  to  the  heart  and  those  that  can  be  classed  as 
vasorribtor. 

The  heart  is  exercised  whenever  the  skeletal  muscles  are 
actively  used  and  the  demand  upon  it  is  roughly  pro- 
portional to  the  intensity  of  the  effort.  There  is  no 
other  way  to  give  this  organ  vigorous  use.  If  it  is  sound 
in  the  beginning  it  responds  to  training  like  any  other 
muscle,  growing  somewhat  more  massive  and  much 
more  hardy  with  exercise.  It  is  to  be  noted  that  a 
heart  may  become  larger  than  normal  in  two  different 
cases:  the  increase  may  be  in  the  thickness  of  its  walls  or 
in  the  volume  of  its  cavities.  The  first  is  hypertrophy, 
the  second  dilation.  A  hypertrophied  heart  is  usually  of 
exceptional  strength,  while  dilation  without  reinforcement 
of  the  walls  is  an  undesirable  change.  Some-  evidence 
has  been  brought  forth  to  show  that  athletes'  hearts  do 
not  hold  out  well  in  later  life,  but  this  is  denied.  It  lies 


SOME    MATTERS    OF    HYGIENE  391 

rather  outside  the  present  argument  which  is  in  support 
of  moderate  rather  than  athletic  activity. 

The  vasomotor  reactions  which  attend  muscular  con- 
traction are  perhaps  sufficiently  clear.  The  vessels  of 
the  muscles  are  dilated  and  so  are  those  of  the  skin .  There 
is  probably  an  offsetting  constriction  in  the  digestive  tract. 
The  adjustment  becomes  more  positive  and  timely  with 
practice.  The  sedentary  person  lacks  the  capacity  to 
make  this  prompt  adaptive  change  in  the  distribution  of 
the  blood.  His  vasomotor  system  lacks  resilience  and 
it  cannot  be  relied  upon  to  make  strong  corrective  reac- 
tions in  the  interest  of  health. 

The  vasomotor  effect  of  a  cold  bath  is  of  a  similar 
nature.  When  the  skin  is  chilled,  the  blood  is  sent  to  the 
internal  organs  in  increased  quantity.  This  primary 
diversion  is  the  reverse  of  that  when  exercise  is  begun. 
But  it  is  followed  by  a  hardening  of  the  muscles  ac- 
companied by  a  heightened  metabolism.  The  heat- 
production  of  the  body  is  augmented  and  when  the 
subject  leaves  the  bath  and  begins  to  rub  down  there  is 
surplus  heat  to  be  thrown  off.  The  vasomotor  reaction 
is  then  in  the  same  direction  as  in  exercise  and  the 
occasion  is  really  the  same.  * 

Muscular  contractions  directly  promote  the  movement 
of  the  blood  aside  from  the  enlistment  of  the  heart  in 
their  support,  provided  only  that  they  are  rhythmic 
rather  than  sustained.  When  a  muscle  grows  tense,  it 
thrusts  the  blood  out  of  its  own  veins.  Other  veins  are 
caught  and  squeezed  between  neighboring  muscles  or 
between  muscles  and  the  skin.  The  veins  of  the  ex- 
tremities, in  which  such  action  is  most  marked,  are 
provided  with  simple  valves  so  placed  as  to  allow  no 
backward  movement  of  blood  toward  the  capillaries. 
Hence  the  emptying  of  these  veins  always  drives  blood 
in  the  direction  of  the  heart  while  they  refill  from  the 
tissues.  This  accelerating  influence  is  lost  when  long- 
sustained  contractions  are  made,  as  in  carrying  a  suit 
case. 


392  HUMAN    PHYSIOLOGY 

The  hastening  of  the  blood-flow  by  the  pressure  of 
contracting  muscles  is  a  kind  of  massage  and  its  effect  is 
extended  to  the  contents  of  the  lymphatics.  The  fact 
has  been  noticed  in  Chapter  XVII.  There  is  every  rea- 
son to  believe  that  it  assists  in  the  removal  of  waste  and 
promotes  the  nutrition  of  the  regions  where  the  condition 
is  operative. 

The  beneficial  results  of  muscular  activity  so  far  as 
they  are  manifest  in  the  digestive  system  are  indirect 
but  important.  The  immediate  consequence  of  activity 
must  always  be  a  withdrawal  of  blood  from  the  alimen- 
tary canal.  This  cannot,  in  itself,  be  other  than  a 
hindrance  to  the  processes  going  on  there.  It  is  likely 
that  excessive  perspiration  leads  to  a  shrinkage  in  the 
volume  of  the  juices.  But  in  spite  of  these  unfavorable 
features  we  know  that  the  net  outcome  is  in  favor  of  the 
man  who  takes  a  fair  amount  of  exercise.  To  some 
extent  his  digestion  may  be  helped  by  the  actual  agitation 
of  the  canal.  A  far  more  significant  reward  is  the  sharp 
appetite  which  usually  presages  a  successful  disposition  of 
the  meal. 

The  Fundamentals  of  Sex  Hygiene. — Here  is  a  matter 
which  is  much  more  widely  and  freely  treated  now  than 
it  was  a  few  years  ago.  There  must  always  be  an  in- 
clination to  reticence  on  the  subject  and  for  the  best  of 
reasons.  The  abnormal  side  of  sexual  life  is  abhorrent 
to  normal  individuals.  The  normal  side  is  limited  to  the 
sacred  intimacy  of  marriage  and  should  remain  inviolate. 
But  there  are  a  few  cardinal  principles  of  thinking  and 
conduct  which  it  cannot  be  an  offense  to  publish. 

The  sexual  instinct  in  the  average  man  is  a  compelling 
one.  This  being  true,  it  is  plainly  his  duty  to  see  to  it 
that  it  shall  not  progressively  encroach  upon  the  sphere 
of  other  interests.  This  is  precisely  what  it  will  do  if  the 
line  of  least  resistance  is  followed.  There  are  men 
everywhere  whose  thoughts  revert  to  the  subject  of  sex 
whenever  they  are  free  to  wander.  Their  ideas  of  en- 
joyment and  humor  are  sexual.  It  is  probably  true  of 


SOME    MATTERS    OF    HYGIENE  393 

men  of  stronger  character  that  most  of  them  wish  that 
the  matter  had  not  become  so  obtrusive.  They  regret 
that  they  have  not  more  firmly  confined  it  within  bounds. 
Nothing  else  so  threatens  the  symmetry,  the  efficiency, 
and  the  height  of  attainment  of  a  man's  life. 

Clearly,  then,  it  is  the  part  of  wisdom  to  occupy  one's 
energies  with  many  concerns  which  shall  not  minister 
at  all  to  the  impulses  of  sex.  As  a  man  who  is  sailing  a 
boat  places  himself  so  that  his  weight  shall  tell  against  the 
heeling  effect  of  the  wind  so  one  should  be  at  pains  to 
trim  the  craft  in  which  he  is  making  the  voyage  of  life. 
To  bring  it  upon  an  even  keel  he  must  match  other  forces 
against  that  which  is  always  bearing  him  over  to  one 
side.  To  drop  the  figure,  he  must  set  himself  tasks 
for  the  muscles  and  the  intellect  which  shall  keep  sex 
in  abeyance. 

Appeals  for  sexual  self-mastery  in  the  name  01  good 
taste,  chivalry,  social  justice,  and  even  religion  have 
genuine  power.  But  we  shall  not  set  them  forth  in  this 
place.  We  shall  be  content  to  urge  that  the  earnest 
cultivation  of  varied  interests,  the  finding  of  pleasure  in 
work  and  sport,  the  stimulus  of  friendships,  and  the 
appreciation  of  the  aged  and  the  little  children  as  well 
as  of  our  own  generation  will  usually  insure  the  relega- 
tion of  sex  to  its  own  proper  but  restricted  place.  Habits 
(which  may  quite  as  well  be  mental  as  physical)  which 
extend  its  dominion  are  calculated  to  lead  on  to  the  most 
unhappy  distortion  of  ideals  and  to  the  forfeiture  of  the 
highest  prizes. 

Conclusion. — The  plea  that  has  just  been  made  is  for 
symmetry,  for  right  proportion,  for  balance.  By  an 
extension  of  the  same  teaching  to  all  of  life  we  are  brought 
back  to  the  requisites  of  hygienic  living  which  have  been 
named.  Given  a  body  of  normal  potentialities,  one's  task 
is  to  nourish  it,  to  set  it  to  work  and  play,  to  grant  it  rest 
as  needed,  and  to  provide  it  with  an  environment  favor- 
able to  its  maintenance  and  activities.  Discussion  of  the 
environment  falls  for  the  most  part  outside  a  text-book  of 


394  .HUMAN    PHYSIOLOGY 

physiology.  We  have  touched  upon  one  of  its  aspects  in 
speaking  of  ventilation.  Its  larger  problems  draw  our 
attention  from  the  individual,  who  is  central  in  physi- 
ology, to  the  contact  of  human  beings  in  communities. 
This  is  the  subject  matter  of  works  on  Public  Health. 

It  is  most  desirable  that  the  reader  who  comes  to  the 
end  of  an  account  such  as  has  been  attempted  here, 
dealing  with  the  body  living  by  itself,  pass  on  to  learn 
something  of  preventive  medicine.  He  will  find  the 
story  one  of  absorbing  interest,  rich  in  personalities  and 
courageous  achievement.  He  will  come  to  realize  how 
the  relative  security  of  life  in  our  time  stands  contrasted 
with  its  uncertainty  in  all  earlier  ages  and  how  bright 
are  the  promises  for  the  future. 


SUGGESTIONS  FOR  COLLATERAL  READING 

A  few  books  may  be  named  here  which  will  be  broadly 
useful  as  works  of  reference.  After  these  have  been 
mentioned  we  will  make  a  list  of  sources  relating  to 
specific  topics  in  the  general  order  in  which  they  have  been 
taken  up  in  the  foregoing  chapters.  In  this  there  will 
be  implied  the  author's  acknowledgment  of  his  indebted- 
ness to  these  books  and  articles. 

General  Works 

Text-books  of  physiology  of  a  large  and  authoritative 
kind:  Howell,  " Text-book  of  Physiology,"  Sixth  Edition, 
Philadelphia,  The  W.  B.  Saunders  Company,  1915. 
Starling,  "  Principles  of  Human  Physiology,"  Second 
Edition,  Philadelphia,  Lea  &  Febiger,  1915. 

To  emphasize  anatomy :  Kimber,  "  Anatomy  and  Physi- 
ology for  Nurses,"  Fourth  Edition,  New  York,  Macmil- 
lan,  1914.  Hill,  "Manual  of  Histology  and  Organo- 
graphy,"  Third  Edition,  Philadelphia,  The  W.  B.  Saun- 
ders Company,  1914. 

For  hygiene:  Pyle,  "A  Manual  of  Personal  Hygiene," 
Sixth  Edition,  Philadelphia,  The  W.  B.  Saunders 
Company,  1915.  Fisher  and  Fisk,  "How  to  Live,"  New 
York,  Funk  and  Wagnalls,  1915. 

For  biologic  background:  Sedgwick  and  Wilson, 
"General  Biology,"  New  York,  Holt,  1895. 

For  historical  background:  Foster,  "Lectures  on  the 
History  of  Physiology,"  Cambridge,  England,  The 
University  Press,  1901.  Locy,  "Biology  and  Its  Makers," 
New  York,  Holt,  1908. 

Special  References 

Chapter  I. — On  vivisection:  Keen,  "Animal  Experi- 
mentation and  Medical  Progress,"  Boston,  Houghton, 
1914. 

395 


396  HUMAN    PHYSIOLOGY 

Chapter  II.— For  Van  Helmont,  Foster,  loc.  cit.,  133; 
for  Priestley,  the  same,  237.  For  the  "  Cycle  of  Nitro- 
gen:" Prescott  and  Winslow,  "Elements  of  Water  Bac- 
teriology," Third  Edition,  New  York,  Wiley,  1915,  p.  4. 

Chapter  III. — For  a  parallel  but  fuller  outline  of 
anatomy:  Hough  and  Sedgwick,  "The  Human  Mechan- 
ism," Boston,  Ginn,  1906,  Chapters  II  and  III. 

Chapter  IV. — For  ciliary  reversal:  Parker,  American 
Journal  of  Physiology,  1905,  xiii,  1. 

Chapter  V. — For  fatigue:  Lee,  Pop.  Sci.  Monthly, 
February,  1910.  For  a  full  statement  of  the  modern 
conception  of  energy  transformation  in  skeletal  muscle: 
Martin,  "The  Human  Body,"  Tenth  Edition,  New  York, 
Holt,  1916. 

Chapter  VI. — For  details  of  nervous  tissue:  Lickley, 
"The  Nervous  System,"  New  York,  Longmans,  1912. 

Chapter  VII. — For  an  interesting  discussion  of  reflexes: 
Hough  and  Sedgwick,  loc.  cit.,  Chapter  VII. 

Chapter  VIII. — For  details:  Lickley,  loc.  cit. 

Chapter  IX. — For  a  fascinating  account  of  some  of  the 
evidence  relating  to  localization  in  the  human  brain: 
Thomson,  "Brain  and  Personality,"  New  York,  Dodd, 
Mead,  1907.  (Most  physiologists  are  more  conservative 
than  this  writer.)  Cf.  Loeb,  "Comparative  Physiology 
of  the  Brain,"  New  York,  The  Science  Series,  1900. 

Chapter  X. — Lickley,  loc.  cit.,  Chapter  X. 

Chapter  XL— The  same. 

Chapter  XIL— Pyle,  loc.  cit.,  275-314.  Courtney, 
"The  Conquest  of  Nerves,"  New  York,  Macmillan,  1911. 
Cannon,  "Bodily  Changes  in  Pain,  Hunger,  Fear  and 
Rage,"  New  York,  Appleton,  1915. 

Chapter  XIII. — Kimber,  loc.  cit. 

Chapter  XIV. — For  movements  of  the  stomach: 
Cannon,  "The  Mechanical  Factors  of  Digestion," 
New  York,  Longmans,  1911.  For  a  record  of  a  memor- 
able research  (Beaumont  on  St.  Martin) :  Osier's  essay, 
"A  Backwood  Physiologist,"  in  "An  Alabama  Student, 


SUGGESTIONS    FOR    COLLATERAL    READING        397 

Etc.,"  New  York,  Oxford  University  Press,  American 
Branch,  1909. 

Chapter  XV. — Cannon,  loc.  cit.  Fischer,  "  Physiology 
of  Alimentation,"  New  York,  Wiley,  1907. 

Chapter  XVI. — Martin,  loc.  cit. 

Chapter  XVII. — Hough  and  Sedgwick,  loc.  cit.,  Chap- 
ter IX.  The  physics  of  "trie  circulation  has  perhaps 
never  been  so  clearly  presented  as  by  Foster :  any  edition 
of  his  "  Text-Book,"  New  York,  Macmillan. 

Chapter  XVIII. — Foster,  loc.  cit. 

Chapter  XIX. — Hough  and  Sedgwick,  loc.  cit.,  Chap- 
ters IX,  XVII,  XXI. 

Chapters  XX,  XXI. — Hough  and  Sedgwick,  Chapters 
X,XVII.  Henderson,  "Life  at  Great  Altitudes,"  Yale 
Review,  July,  1914,  p.  759. 

Chapter  XXII.— For  additional  details:  Lusk,  "Ele- 
ments of  the  Science  of  Nutrition,"  Second  Edition, 
Philadelphia,  Saunders,  1909.  Taylor,  "Digestion  and 
Metabolism,"  Philadelphia,  Lea  &  Febiger,  1912. 

Chapter  XXIII. — The  text-books,  especially  Starling. 

Chapter  XXIV. — The  publications  of  the  Carnegie 
Institution. 

Chapter  XXV. — For  an  admirable  summary:  Lusk, 
"The  Fundamental  Basis  of  Nutrition,"  New  Haven, 
Yale  University  Press,  1914. 

Chapter  XXVI. — On  foods  and  dietetics:  Hutchison, 
"Food  and  Dietetics,"  London,  Wood,  1911.  Sherman, 
"Chemistry  of  Food  and  Nutrition,"  New  York, 
Macmillan,  1910.  Jordan,  "Principles  of  Nutrition," 
New  York,  Macmillan,  1911.  Bailey,  "Source,  Chem- 
istry and  Use  of  Food  Products,"  Philadelphia,  Blakis- 
ton,  1914.  For  food  poisoning:  Dieudonne  (Bolduan, 
translator),  "Bacterial  Food  Poisoning,"  New  York, 
Treat,  1909.  Advocating  restricted  diet:  Chittenden, 
"Physiological  Economy  in  Nutrition,"  New  York, 
Stokes,  1904.  In  rebuttal:  Benedict,  American  Journal 
of  Physiology,  1906,  xvi,  p.  409.  Meltzer,  Journal  of 
the  American  Medical  Association,  1907,  xlviii,  p.  655. 


398  HUMAN    PHYSIOLOGY 

Crichton-Browne,  "  Delusions  in  Diet,"  New  York, 
Funk  &  Wagnalls,  1909. 

Chapter  XXVII. — Hough  and  Sedgwick,  loc.  tit., 
Chapter  XII. 

Chapter  XXVIII.     The  text-books. 

Chapter  XXIX. — For  an  advanced  discussion  of 
Mechanism  and  Vitalism:  Haldane,  " Mechanism,  Life 
and  Personality,"  New  York,  Dutton,  1914.  For 
general  hygiene:  Pyle,  loc.  tit.,  various  sections.  Hough 
and  Sedgwick,  loc.  tit.,  Chapters  XVI-XXV.  Galbraith, 
"  Personal  Hygiene  and  Physical  Training  for  Women," 
Philadelphia,  Saunders,  1911.  For  Sex  Hygiene,  Cor- 
bett-Smith,  "The  Problem  of  the  Nations,"  London,  Bale, 
Sons  and  Danielsson,  1914.  For  preventive  medicine: 
Rosenau,  "  Preventive  Medicine  and  Hygiene,  New  York, 
Appleton,  1913.  Sedgwick,  "Principles  of  Sanitary 
Science  and  the  Public  Health,"  New  York,  Macmillan, 
1902.  Kelly,  "Walter  Reed  and  Yellow  Fever,"  New 
York,  McClure,  1906.  Councilman,  "Disease  and  Its 
Causes,"  New  York,  Holt,  1913. 


INDEX 


ABSORPTION,  212-215 
Acapnia,  297 
Accelerators  of  heart,  262 
Acclimatization,  299,  300 
Accommodation,  153 
Acid  in  stomach,  197,  199 
Acidosis,  305 
Adaptation,  20 
Addison's  bronze  disease,  377 
Adrenals    (and    adrenin),    377- 
381 

and  emotions,  378 
Afferent  nerve-fibers,  87,  96 

side    of    vasomotor    control, 

273-275 

Air  chamber,  235 
Alcohol,  171-173,  310 
Alimentary  canal,  174-182 

glycosuria,  307 
All  or  none  principle,  258 
Altitudes,  high,  298-300 
Ameba,  53-54 
Ameboid  movement,  53-57 
Amino-acids,  206,  310-314 
Amputation,  139 
Amylase,  205 
Anatomical  terms,  51 
Anatomy,  48-52 
Ancestors,  45 

Antagonism  of  muscles,  66 
Anticipation,  165 
Antiperistalsis,  210 
Antrum  of  stomach,  193 
Aphasia,  motor,  135 


Apnea,  298 
Arterial  blood,  291 
Arteries,  227,  229 
Arterio-sclerosis,  235,  353 
Association  areas,  132,  133 

units,  105 
Astigmatism,  156 
Auto-intoxication,  353 

measures  against,  354 
Automaticity,  62,  111,  257 
Autonomic  system,  118-120 

BACTERIA,  34,  35,  45,  212,  353 
Beri-beri,  347 
Bernard,  265 
Bile,  206-207 

pigments,  207 
Binocular  vision,  160 
Birth,  244 
Bladder,  gall,  206 

urinary,  318 
Blood,  46,  216-226 

plates,  223 

pressure,  231-234 

quantity,  217 
Body-cavities,  49,  50 
Body-temperature,  365,  366 
Brain,  106-135 
Breathing,  276-287 
Bronchi,  277 
Bundle  of  His,  251,  256 


CAFFEIN,  346 
Caisson  sickness,  295 


399 


400 


INDEX 


Calorimetry,  330-335 

indirect,  332 
Capillaries,  228 

Carbon  dioxid  and  the  respira- 
tory center,  296-298 

monoxid,  293 
Cardiac  muscle,  254,  255 
Cathartics,  360 
Cecum,  210 
Cell  theory,  38 
Cells,  38 

ciliated,  57 

free-living,  42 
Cellulose,  211 
Cerebellum,  113,  114 
Cerebrum,  120-135 
Chocolate,  173 
Circulation,  227-244,  260-275 

before  birth,  241-244 

pulmonary,  229,  238 
Coagulation,  223-226 
Cochlea,  147 
Coffee,  173 
Colds,  269,  270 
Collapse  of  lung,  279 
Colon,  178,  207,  356 
Color-blindness,  158 
Compressed  air,  294,  295 
Conduction  of  heat,  369 
Congestions,  268 
Conservation  of  energy,  16.  32 
Constipation,  359 
Contraction,  43,  53-80 

simple,  70 

summated,  71 

tetanic,  72 
Convergence,  161 
Cooking,  350 
Coordination,  22-24,  100 
Cord,  spinal,  85 
Corpuscles,  217,  219 
Cranial  blood-supply,  270-273 


Cranial  nerves,  109 

Cretins,  376 

Cycle  of  nitrogen,  35 

DEAD-SPACE,  286 
Decerebrate  animals,  121-124 

child,  125 
Defecation,  211 
Deficiency  diseases,  347-351 
Degeneration  of  nerve-fibers,  89 
Degradation  of  energy,  25 
Dendrites,  89 
Depressor  reaction,  274 
Dextrose,  306 
Diabetes,  309 
Diapedesis,  56  (Fig.) 
Diaphragm,  283 
Dietetics,  340-351 
Digestion,  183,  184 

fat,  200,  204 

pancreatic,  204 

peptic,  199,200 

salivary,  189,  196 

tryptic,  205 
Digestive  juices,  184 
Dog,  decerebrate,  124 
Dreaming,  171 
Drugs,  387 
Ductus  arteriosus,  242 

EAR,  144 

inner,  116,  147 

middle,  145,  146 
Economy  of  muscle,  79 
Effectors,  21,  106 
Elbow,  133 

Emotion,  166,  378-381 
Emotional  glycosuria,  325 
Emulsification,  187 
End-plate,  motor,  82,  91 
Energy  of  metabolism,  329 

of  skeletal  muscle,  77 


INDEX 


401 


Enzymes,  184-187 
Epithelium,  41 

ciliated,  58 
Equilibrium,  113-118 
Erepsin,  206 

Escape  from  inhibition,  262 
Esophagus,  177 
Esquimaux,  340,  345 
Eustachian  tube,  145 
Excretion,  315-325 
Exercise,  387-391 

temperature  control  in,  371, 

372 

Expiration,  285 
Extractives,  organic,  345 
Eye,  149-162 
Eye-ball,  150-152 
Eye-muscles,  93 
Eyes  and  equilibrium,  115 

FAR-SIGHT,  154,  155 
Fatigue,  muscular,  73-77 

nervous,  166-169 

neuromuscular,  90,  91 

substances,  75 
Feces,  211 
Fermentation,  200 
Fever,  372 
Fibrin,  224 
Fibrinogen,  224 
Fish,  121 
Food,  33,  183 

accessories,  345-351 

poisoning,  357 

proportions  of,  335,  336 
Foramen  ovale,  242 
Forced  breathing,  297 
Fovea,  157,  161 
Frog,  122 
Fuel  values,  329 
Fuels,  25-27 
Fundus  of  stomach,  192 

26 


GANGLIA,  96 
Gastric  juice,  197 
Gastrocnemius,  68-77 
Gelatin,  311 
Glands,  digestive,  180 

structure  of,  181,  182 
Glottis,  285 
Glycogen,  306,  307 
Glycosuria,  alimentary,  307 

emotional,  325 
Goiter,  375 

Graham,  Sylvester,  349 
Gravity  and   circulation,    235- 

237 

Gray  matter,  87,  98 
Growth,  314,  324 


HABIT,  101,  163 
Hales,  Stephen,  16,  232 
Hallucinations,  136 
Harvey,  William,  15,  45 
Hearing,  143-148 
Heart,  245-259 

block,  253 

cycle,  249-253 

effect  of  exercise  on,  390 

nerves,  260-264 

sounds,  253 

valves,  247,  248 
Hemoglobin,  220 
Hemophilia,  224 
Homothermous  animals,  364 
Hormones,  205,  374-383 
Humidity,  367,  368 
Humors,  four,  375 
Hunger,  194 
Hygiene,  general,  384-394 

of  breathing,  301  302 

of  exercise,  387-391 

of  nervous  system,  163-173 

of  nutrition,  352-363 


402 


INDEX 


Hyperglycemia,  307 
Hypophysis,  381 

IDIOSYNCRASIES,  359 

Indestructibility  of  matter,  16 

Indigestion,  167,  352 

Infant,  102 

Infusoria,  42 

Inhibition,  22,  103,  134,  164 

of  heart,  261 

Inner  ear  and  equilibrium,  116 
Insect's  muscle,  71 
Inspiration,  281-284 
Intercellular  matter,  39 
Intermittent  flow  in  arteries,  234 
Internal  secretions,  374-383 
Intestine,  large,  178 

small,  177 
Iron,  343 
Isodynamic  quantities,  341 

JAMES,  William,  on  habits,  164 
Judgments,  visual,  160-162 

KIDNEYS,  317,  318 

LABYRINTH,  116 
Lactic  acid,  75,  355 
Language,  134,  135 
Latent  heat,  330,  366,  367 
Leucocytes,  55,  56,  223 
Liebig,  308 
Light,  29,  149 
Liminal  distance,  141 
Lipase^  gastric,  200 

pancreatic,  205 

Localization,  cerebral,  125-135 
Lungs,  228,  277 
Lymph,  47,  239 
Lymphatics,  215,  239-241 
Lymph-nodes,  241 


MAINTENANCE,  21 

Marrow,  red,  221 

Mastication,  188 

Meat,  355 

Mechanism,  19,  384 

Medulla,  110-113 

Meninges,  85 

Mental  states,  386 

and  indigestion,  353 

Mesentery,  178,  179 

Metabolism,  303-314 
and  surface,  338 
conditions  affecting,  336-339 
estimation  of,  322,  330-335 
of  amino-acids,  310-314 
of  carbohydrates,  306-309 
of  fats,  303-305 

Microscope,  37 

Mineral  matter,  342 

Mitral  valve,  247 

Motor  areas,  127,  128 
end-plate,  82,  91 

Mountain-sickness,  299 

Mucous  membrane,  181 

Muller,  Johannes,  16 

Miillerian  doctrine,  137,  138 

Muscle,  60-80 
skeletal,  65-80 
smooth,  61 

Muscular  exercise,  387-391 

NEAR-SIGHT,  154 
Nerve,  82 

cell,  88 

fibers,  82 

afferent,  87,  96 
efferent,  85 

impulse,  83,  84 

resistant  to  fatigue,  84 

roots,  85 
Nerves,  cranial,  109 

spinal,  85 


INDEX 


403 


Neurasthenia,  167 
Neuritis,  multiple,  348 
Neuromuscular  efficiency,  388, 
389 

fatigue,  90,  91 

unit,  91,  92 
Neuron,  89 
Nitrogenous  equilibrium,  323 

metabolism,  310-314 
Nose,  278 
Nucleus,  39 
Nystagmus,  117 

OBESITY,  361 

control  of,  362 
Odors,  142,  143 
Omentum,  great,  179 
Organic  compounds,  25 
Ovaries,  381,  382 
Oxidation,  25-27 

in  muscle,  78 

Oxygen,  breathing,  293,  294 
Oxy hemoglobin,  221 

PANCREAS,  204,  308,  309 
Pancreatic  juice,  204 
Parathyroids,  377 
Penniform  muscle,  68 
Pepsin,  200 
Pericardium,  246 
Peristalsis,  190,  203 
Peritoneum,  179 
Phagocytosis,  55 
Phosphates  in  urine,  319 
Photosynthesis,  28 
Phrenology,  126 
Pigeons  and  rice  diet,  348 

decerebrate,  123 
Pineal  body,  383 
Placenta,  242 
Plants  and  animals,  30 
Plasma,  217 
Plethysmograph,  271 


Pneumothorax,  280 
Portal  system,  237,  238 
Post-ganglionic  fibers,  119 
Posture  and  temperature,  371 
Pre-ganglionic  fibers,  119 
Presbyopia,  153,  154 
Pressor  reaction,  274 
Preventive  medicine,  394 
Priestley,  Joseph,  28 
Protein  metabolism,  310-314 

synthesis,  31,  32 
Protoplasm,  39 
Psycho-reflexes,  103,  104 
Ptomains,  357 
Ptyalin,  189 

Pulmonary  circulation,  229,  238 
Pulse,  259 

RADIATION,  369 
Reaction  time,  105 
Receptors,  21,  94,  136-162 
Reciprocal  innervation,  92,  93 
Reflex  time,  105 
Reflexes,  94-105 
Regeneration  of  nerve  fibers,  90 
Rennin,  199 
Reproduction,  44 
Resistance,  synaptic,  99—102 
Respiration  chamber,  326-329 
Respiratory  center,  110-112 
Respired  air,  287,  288 
Rest,  169 
Retina,  157-159 

and  brain,  130,  131 
Ribs,  282 

Rice,  polished,  348 
Rods  and  cones,  157 
Roots,  nerve,  85 

SALIVA,  188-190,  196 
Sarcolemma,  67 
Scurvy,  347 


404 


INDEX 


Sea  anemone,  59,  60 
Secretagogues,  198,  346 
Secretin,  204,  374 
Secretion,  103 

Segmentation,  rhythmic,  203 
Semilunar  valves,  248 
Sensations,  136-162 

common  or  general,  139 

kinesthetic,  140 

temperature,  140 
Sensory  areas,  cerebral,  129-132 
Serum,  224 
Sex,  46 

hygiene,  392,  393 
Sigmoid  flexure,  210 
Skin,  140 
Sleep,  169-171 
Smell,  142,  143 
Speech  center,  134 
Sphincter,  cardiac,  195 

pyloric,  195 

Splanchnic  nerves,  265,  266 
Spleen,  222,  382,  383 
Stereoscopic  principle,  160 
Stimuli,  24 

Stokes^ Adams  disease,  256,  257 
Stomach,  177,  193-200 

movements  of,  191—196 
Strength  of  desperation,  379 
Sulphates  in  urine,  319 
Swallowing,  190,  191 
Sweat,  315 

glands,  321 

Sympathetic  system,  119 
Synapses,  98 

TASTE,  141 

Tea,  173 

Teeth,  preservation  of,  362,  363 

Temperature  maintenance,  364- 

373 
Tendons,  65 


Testes,  381,  382 
Theobromin,  346 
Thermogenesis,  366 
Thermotaxis,  366 
Thoracic  duct,  240 
Thrombin,  224,  225 
Thymus,  383 
Thyroid,  375-377 
Tidal  air,  285 
Tissues,  39,  40 

connective,  41,  60 

contractile,  41,  66 

epithelial,  41 

nervous,  41,  82,  88 
Tone,  63,  64 
Tongue,  141 
Trachea,  277 
Treppe,  74 
Tricuspid  valve,  247 
Trypsin,  205 

UNDEREATING,  356,  357 
Urea,  35,  313 
Ureter,  318 
Urethra,  318 
Urine,  318-325 

VAGUS  NERVE,  261 
Valve  action  of  synapse,  99 
Valves  of  heart,  247,  248 
Van  Helmont,  30 
Vasoconstrictors,  265 
Vasodilators,  266,  267 
Vasomotor  center,  112 
mechanism,  264,  275 

and  bathing,  391 

and  exercise,  391 
Veins,  227 
Velocity     of     blood-flow,    230, 

231 

Venous  blood,  291 
Ventilation,  289,  290 


INDEX 


405 


Villi  213  Vivisection,  17,  18 

Virchow,  45  Vomiting,  196 
Vital  capacity,  286,  287 

resistance,  270  WATER,  315-317,  341,  342 

Vitalism,  19,  384,  385  White  corpuscles,  223 
Vitamins,  346-351  matter,  87 


14  DAY  USE 

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